Fix more "may be used initialized" warnings from GCC 7

Fixes the following GCC 7 warnings:
```
core/cowdata.h:269:47: warning: 'alloc_size' may be used uninitialized in this function [-Wmaybe-uninitialized]
core/error_macros.h:163:26: warning: 'nearest_point' may be used uninitialized in this function [-Wmaybe-uninitialized]
core/image.cpp:1579:5: warning: 'colormap_size' may be used uninitialized in this function [-Wmaybe-uninitialized]
core/image.cpp:1582:12: warning: 'size_height' may be used uninitialized in this function [-Wmaybe-uninitialized]
core/image.cpp:1590:23: warning: 'size_width' may be used uninitialized in this function [-Wmaybe-uninitialized]
core/image.cpp:1599:29: warning: 'pixel_size' may be used uninitialized in this function [-Wmaybe-uninitialized]
core/math/face3.cpp:207:15: warning: 'tri_max' may be used uninitialized in this function [-Wmaybe-uninitialized]
core/math/face3.cpp:209:15: warning: 'tri_min' may be used uninitialized in this function [-Wmaybe-uninitialized]
drivers/gles3/rasterizer_scene_gles3.cpp:665:22: warning: 'best_used_frame' may be used uninitialized in this function [-Wmaybe-uninitialized]
drivers/gles3/rasterizer_storage_gles3.cpp:865:27: warning: 'blit_target' may be used uninitialized in this function [-Wmaybe-uninitialized]
drivers/gles3/rasterizer_storage_gles3.cpp:980:29: warning: 'blit_target' may be used uninitialized in this function [-Wmaybe-uninitialized]
drivers/gles3/shader_gles3.h:122:9: warning: '<anonymous>.ShaderGLES3::Version::frag_id' may be used uninitialized in this function [-Wmaybe-uninitialized]
drivers/gles3/shader_gles3.h:122:9: warning: '<anonymous>.ShaderGLES3::Version::id' may be used uninitialized in this function [-Wmaybe-uninitialized]
drivers/gles3/shader_gles3.h:122:9: warning: '<anonymous>.ShaderGLES3::Version::vert_id' may be used uninitialized in this function [-Wmaybe-uninitialized]
editor/plugins/script_editor_plugin.cpp:1980:31: warning: 'se' may be used uninitialized in this function [-Wmaybe-uninitialized]
editor/scene_tree_dock.cpp:840:30: warning: 'new_node' may be used uninitialized in this function [-Wmaybe-uninitialized]
editor/spatial_editor_gizmos.cpp:4259:9: warning: 'a1' may be used uninitialized in this function [-Wmaybe-uninitialized]
editor/spatial_editor_gizmos.cpp:4259:9: warning: 'lll' may be used uninitialized in this function [-Wmaybe-uninitialized]
editor/spatial_editor_gizmos.cpp:4259:9: warning: 'lul' may be used uninitialized in this function [-Wmaybe-uninitialized]
editor/spatial_editor_gizmos.cpp:4260:9: warning: 'a2' may be used uninitialized in this function [-Wmaybe-uninitialized]
editor/spatial_editor_gizmos.cpp:4261:9: warning: 'a3' may be used uninitialized in this function [-Wmaybe-uninitialized]
editor/spatial_editor_gizmos.cpp:4265:3: warning: 'enable_lin' may be used uninitialized in this function [-Wmaybe-uninitialized]
editor/spatial_editor_gizmos.cpp:4294:3: warning: 'enable_ang' may be used uninitialized in this function [-Wmaybe-uninitialized]
editor/spatial_editor_gizmos.cpp:4311:34: warning: 'll' may be used uninitialized in this function [-Wmaybe-uninitialized]
editor/spatial_editor_gizmos.cpp:4311:34: warning: 'ul' may be used uninitialized in this function [-Wmaybe-uninitialized]
scene/3d/voxel_light_baker.cpp:1655:47: warning: 'cone_dirs' may be used uninitialized in this function [-Wmaybe-uninitialized]
scene/3d/voxel_light_baker.cpp:1656:73: warning: 'cone_weights' may be used uninitialized in this function [-Wmaybe-uninitialized]
scene/gui/texture_progress.cpp:181:6: warning: 'cp' may be used uninitialized in this function [-Wmaybe-uninitialized]
scene/gui/texture_progress.cpp:181:6: warning: 'cq' may be used uninitialized in this function [-Wmaybe-uninitialized]
servers/physics/shape_sw.cpp:1056:19: warning: 'support_max' may be used uninitialized in this function [-Wmaybe-uninitialized]
```

3 files changed, 13 insertions, 8 deletions

diff --git a/core/cowdata.h b/core/cowdata.h
index 9e75d4ed55..54ece4c856 100644
--- a/core/cowdata.h
+++ b/core/cowdata.h
@@ -87,7 +87,10 @@ private:
 #if defined(_add_overflow) && defined(_mul_overflow)
 		size_t o;
 		size_t p;
-		if (_mul_overflow(p_elements, sizeof(T), &o)) return false;
+		if (_mul_overflow(p_elements, sizeof(T), &o)) {
+			*out = 0;
+			return false;
+		}
 		*out = next_power_of_2(o);
 		if (_add_overflow(o, static_cast<size_t>(32), &p)) return false; //no longer allocated here
 		return true;
diff --git a/core/image.cpp b/core/image.cpp
index e2b56c51dc..ee3f0121f9 100644
--- a/core/image.cpp
+++ b/core/image.cpp
@@ -1471,7 +1471,8 @@ void Image::create(int p_width, int p_height, bool p_use_mipmaps, Format p_forma
 
 void Image::create(const char **p_xpm) {
 
-	int size_width, size_height;
+	int size_width = 0;
+	int size_height = 0;
 	int pixelchars = 0;
 	mipmaps = false;
 	bool has_alpha = false;
@@ -1487,8 +1488,8 @@ void Image::create(const char **p_xpm) {
 	int line = 0;
 
 	HashMap<String, Color> colormap;
-	int colormap_size;
-	uint32_t pixel_size;
+	int colormap_size = 0;
+	uint32_t pixel_size = 0;
 	PoolVector<uint8_t>::Write w;
 
 	while (status != DONE) {
diff --git a/core/math/face3.cpp b/core/math/face3.cpp
index aa46fde7f7..8366137131 100644
--- a/core/math/face3.cpp
+++ b/core/math/face3.cpp
@@ -202,11 +202,12 @@ bool Face3::intersects_aabb(const AABB &p_aabb) const {
 	{                                                              \
 		real_t aabb_min = p_aabb.position.m_ax;                    \
 		real_t aabb_max = p_aabb.position.m_ax + p_aabb.size.m_ax; \
-		real_t tri_min, tri_max;                                   \
-		for (int i = 0; i < 3; i++) {                              \
-			if (i == 0 || vertex[i].m_ax > tri_max)                \
+		real_t tri_min = vertex[0].m_ax;                           \
+		real_t tri_max = vertex[0].m_ax;                           \
+		for (int i = 1; i < 3; i++) {                              \
+			if (vertex[i].m_ax > tri_max)                          \
 				tri_max = vertex[i].m_ax;                          \
-			if (i == 0 || vertex[i].m_ax < tri_min)                \
+			if (vertex[i].m_ax < tri_min)                          \
 				tri_min = vertex[i].m_ax;                          \
 		}                                                          \
                                                                    \