Fix various warnings: [-Waddress], [-Wpointer-arith], [-Wwrite-strings], [-Wreturn-local-addr] and more

Fixes the following GCC 5 warnings:
```
core/os/file_access.cpp:49:19: warning: the address of 'FileAccess::create_func' will always evaluate as 'true' [-Waddress]
servers/audio_server.cpp:192:70: warning: comparison with string literal results in unspecified behaviour [-Waddress]

drivers/gles2/rasterizer_storage_gles2.cpp:4095:90: warning: NULL used in arithmetic [-Wpointer-arith]

modules/gdnative/register_types.cpp:237:3: warning: deprecated conversion from string constant to 'char*' [-Wwrite-strings]
platform/android/export/export.cpp:207:1: warning: deprecated conversion from string constant to 'char*' [-Wwrite-strings]

modules/gdscript/gdscript.h:150:67: warning: returning reference to temporary [-Wreturn-local-addr]
servers/physics_2d/collision_object_2d_sw.h:119:56: warning: returning reference to temporary [-Wreturn-local-addr]
servers/physics_2d/collision_object_2d_sw.h:123:56: warning: returning reference to temporary [-Wreturn-local-addr]
servers/physics_2d/collision_object_2d_sw.h:127:50: warning: returning reference to temporary [-Wreturn-local-addr]
servers/physics_2d/collision_object_2d_sw.h:131:52: warning: returning reference to temporary [-Wreturn-local-addr]

editor/plugins/skeleton_editor_plugin.cpp:34:36: warning: extra tokens at end of #include directive
modules/bullet/bullet_types_converter.cpp:31:9: warning: #pragma once in main file

editor/import/editor_scene_importer_gltf.cpp:1996:51: warning: name lookup of 'i' changed
modules/visual_script/visual_script_property_selector.cpp:402:45: warning: name lookup of 'E' changed
scene/gui/tree.cpp:1268:25: warning: name lookup of 'i' changed
scene/resources/visual_shader.cpp:808:32: warning: name lookup of 'i' changed
```

1 files changed, 4 insertions, 4 deletions

diff --git a/scene/resources/visual_shader.cpp b/scene/resources/visual_shader.cpp
index f12ea8f2bb..96b9cfa137 100644
--- a/scene/resources/visual_shader.cpp
+++ b/scene/resources/visual_shader.cpp
@@ -796,10 +796,10 @@ Error VisualShader::_write_node(Type type, StringBuilder &global_code, StringBui
 				val.basis.transpose();
 				inputs[i] = "n_in" + itos(node) + "p" + itos(i);
 				Array values;
-				for (int i = 0; i < 3; i++) {
-					values.push_back(val.basis[i].x);
-					values.push_back(val.basis[i].y);
-					values.push_back(val.basis[i].z);
+				for (int j = 0; j < 3; j++) {
+					values.push_back(val.basis[j].x);
+					values.push_back(val.basis[j].y);
+					values.push_back(val.basis[j].z);
 				}
 				values.push_back(val.origin.x);
 				values.push_back(val.origin.y);