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, 10 insertions, 8 deletions

diff --git a/drivers/gles3/rasterizer_scene_gles3.cpp b/drivers/gles3/rasterizer_scene_gles3.cpp
index 14c436fd00..2803f3371b 100644
--- a/drivers/gles3/rasterizer_scene_gles3.cpp
+++ b/drivers/gles3/rasterizer_scene_gles3.cpp
@@ -653,7 +653,7 @@ bool RasterizerSceneGLES3::reflection_probe_instance_begin_render(RID p_instance
 
 	int best_free = -1;
 	int best_used = -1;
-	uint64_t best_used_frame;
+	uint64_t best_used_frame = 0;
 
 	for (int i = 0; i < reflection_atlas->reflections.size(); i++) {
 		if (reflection_atlas->reflections[i].owner == RID()) {
diff --git a/drivers/gles3/rasterizer_storage_gles3.cpp b/drivers/gles3/rasterizer_storage_gles3.cpp
index 58c0a104c1..4e1fb46f48 100644
--- a/drivers/gles3/rasterizer_storage_gles3.cpp
+++ b/drivers/gles3/rasterizer_storage_gles3.cpp
@@ -730,7 +730,7 @@ void RasterizerStorageGLES3::texture_set_data(RID p_texture, const Ref<Image> &p
 		}
 	};
 
-	GLenum blit_target;
+	GLenum blit_target = GL_TEXTURE_2D;
 
 	switch (texture->type) {
 		case VS::TEXTURE_TYPE_2D: {
@@ -948,7 +948,7 @@ void RasterizerStorageGLES3::texture_set_data_partial(RID p_texture, const Ref<I
 	Image::Format real_format;
 	Ref<Image> img = _get_gl_image_and_format(p_sub_img, p_sub_img->get_format(), texture->flags, real_format, format, internal_format, type, compressed, srgb);
 
-	GLenum blit_target;
+	GLenum blit_target = GL_TEXTURE_2D;
 
 	switch (texture->type) {
 		case VS::TEXTURE_TYPE_2D: {
diff --git a/drivers/gles3/shader_gles3.h b/drivers/gles3/shader_gles3.h
index 9db4942163..0d360e8453 100644
--- a/drivers/gles3/shader_gles3.h
+++ b/drivers/gles3/shader_gles3.h
@@ -128,11 +128,13 @@ private:
 		Vector<GLint> texture_uniform_locations;
 		uint32_t code_version;
 		bool ok;
-		Version() {
-			code_version = 0;
-			ok = false;
-			uniform_location = NULL;
-		}
+		Version() :
+				id(0),
+				vert_id(0),
+				frag_id(0),
+				uniform_location(NULL),
+				code_version(0),
+				ok(false) {}
 	};
 
 	Version *version;