Fix warnings for comparison between signed and unsigned integers [-Wsign-compare]

Also turn off -Wsign-compare warnings in the future, we do not consider them important.

Fixes the following GCC 5 warnings:
```
core/node_path.cpp:279:24: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
core/oa_hash_map.h:169:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
core/oa_hash_map.h:314:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
drivers/gles2/shader_gles2.cpp:985:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
drivers/gles3/rasterizer_storage_gles3.cpp:1075:25: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
drivers/pulseaudio/audio_driver_pulseaudio.cpp:343:34: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
editor/editor_plugin.cpp:525:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
editor/editor_properties_array_dict.cpp:747:25: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
editor/plugins/spatial_editor_plugin.cpp:2078:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
editor/plugins/spatial_editor_plugin.cpp:4096:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
editor/plugins/sprite_editor_plugin.cpp:100:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
modules/cvtt/image_compress_cvtt.cpp:122:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
modules/cvtt/image_compress_cvtt.cpp:134:77: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
modules/cvtt/image_compress_cvtt.cpp:339:25: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
modules/etc/image_etc.cpp:222:34: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
modules/gdnative/register_types.cpp:242:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
modules/gdnative/register_types.cpp:258:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
modules/opensimplex/simplex_noise.cpp:200:13: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
modules/opensimplex/simplex_noise.cpp:222:13: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
modules/opensimplex/simplex_noise.cpp:246:13: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
platform/android/export/export.cpp:1085:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
platform/android/export/export.cpp:1489:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
platform/android/export/export.cpp:1623:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
platform/iphone/export/export.cpp:206:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
platform/iphone/export/export.cpp:356:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
platform/iphone/export/export.cpp:406:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
platform/iphone/export/export.cpp:493:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
scene/3d/audio_stream_player_3d.cpp:420:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
scene/resources/audio_stream_sample.cpp:565:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
scene/resources/audio_stream_sample.cpp:571:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
servers/audio/audio_rb_resampler.cpp:156:36: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
```

The following warnings were not fixed, as they implied casting for no gain:
```
core/io/packet_peer.cpp:228:38: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
core/io/resource_format_binary.cpp:109:11: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
drivers/gles2/rasterizer_scene_gles2.cpp:144:57: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
drivers/unix/file_access_unix.cpp:249:46: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
scene/3d/voxel_light_baker.cpp:889:14: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
scene/3d/voxel_light_baker.cpp:1020:14: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
scene/3d/voxel_light_baker.cpp:1154:14: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
scene/3d/voxel_light_baker.cpp:2255:38: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
scene/resources/bit_mask.cpp:336:25: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
servers/audio/audio_stream.cpp:141:49: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
servers/audio/audio_stream.cpp:150:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
servers/audio/audio_stream.cpp:154:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
servers/audio_server.cpp:86:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
servers/audio_server.cpp:89:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
```

1 files changed, 3 insertions, 3 deletions

diff --git a/platform/android/export/export.cpp b/platform/android/export/export.cpp
index 6ec7d27464..4fba811cc2 100644
--- a/platform/android/export/export.cpp
+++ b/platform/android/export/export.cpp
@@ -1082,7 +1082,7 @@ public:
 		r_options->push_back(ExportOption(PropertyInfo(Variant::BOOL, "screen/support_large"), true));
 		r_options->push_back(ExportOption(PropertyInfo(Variant::BOOL, "screen/support_xlarge"), true));
 
-		for (int i = 0; i < sizeof(launcher_icons) / sizeof(launcher_icons[0]); ++i) {
+		for (unsigned int i = 0; i < sizeof(launcher_icons) / sizeof(launcher_icons[0]); ++i) {
 			r_options->push_back(ExportOption(PropertyInfo(Variant::STRING, launcher_icons[i].option_id, PROPERTY_HINT_FILE, "*.png"), ""));
 		}
 
@@ -1486,7 +1486,7 @@ public:
 
 			if (file == "res/drawable/icon.png") {
 				bool found = false;
-				for (int i = 0; i < sizeof(launcher_icons) / sizeof(launcher_icons[0]); ++i) {
+				for (unsigned int i = 0; i < sizeof(launcher_icons) / sizeof(launcher_icons[0]); ++i) {
 					String icon_path = String(p_preset->get(launcher_icons[i].option_id)).strip_edges();
 					if (icon_path != "" && icon_path.ends_with(".png")) {
 						FileAccess *f = FileAccess::open(icon_path, FileAccess::READ);
@@ -1620,7 +1620,7 @@ public:
 			APKExportData ed;
 			ed.ep = &ep;
 			ed.apk = unaligned_apk;
-			for (int i = 0; i < sizeof(launcher_icons) / sizeof(launcher_icons[0]); ++i) {
+			for (unsigned int i = 0; i < sizeof(launcher_icons) / sizeof(launcher_icons[0]); ++i) {
 				String icon_path = String(p_preset->get(launcher_icons[i].option_id)).strip_edges();
 				if (icon_path != "" && icon_path.ends_with(".png") && FileAccess::exists(icon_path)) {
 					Vector<uint8_t> data = FileAccess::get_file_as_array(icon_path);