Add comments at the top of each built-in shader to ease debugging

When a shader error is printed about a built-in shader, the origin
of the shader will now be recognizable immediately by looking at
the top of the printed shader code.

servers/rendering/renderer_rd/forward_clustered/scene_shader_forward_clustered.cpp

@@ -683,6 +683,8 @@ void SceneShaderForwardClustered::init(RendererStorageRD *p_storage, const Strin
 		default_shader = storage->shader_allocate();
 		storage->shader_initialize(default_shader);
 		storage->shader_set_code(default_shader, R"(
+// Default 3D material shader (clustered).
+
 shader_type spatial;
 
 void vertex() {
@@ -712,6 +714,8 @@ void fragment() {
 		storage->shader_initialize(overdraw_material_shader);
 		// Use relatively low opacity so that more "layers" of overlapping objects can be distinguished.
 		storage->shader_set_code(overdraw_material_shader, R"(
+// 3D editor Overdraw debug draw mode shader (clustered).
+
 shader_type spatial;
 
 render_mode blend_add, unshaded;

servers/rendering/renderer_rd/forward_mobile/scene_shader_forward_mobile.cpp

@@ -673,6 +673,8 @@ void SceneShaderForwardMobile::init(RendererStorageRD *p_storage, const String p
 		default_shader = storage->shader_allocate();
 		storage->shader_initialize(default_shader);
 		storage->shader_set_code(default_shader, R"(
+// Default 3D material shader (mobile).
+
 shader_type spatial;
 
 void vertex() {
@@ -701,6 +703,8 @@ void fragment() {
 		storage->shader_initialize(overdraw_material_shader);
 		// Use relatively low opacity so that more "layers" of overlapping objects can be distinguished.
 		storage->shader_set_code(overdraw_material_shader, R"(
+// 3D editor Overdraw debug draw mode shader (mobile).
+
 shader_type spatial;
 
 render_mode blend_add, unshaded;

servers/rendering/renderer_rd/renderer_canvas_render_rd.cpp

@@ -2575,6 +2575,8 @@ RendererCanvasRenderRD::RendererCanvasRenderRD(RendererStorageRD *p_storage) {
 		storage->shader_initialize(default_canvas_group_shader);
 
 		storage->shader_set_code(default_canvas_group_shader, R"(
+// Default CanvasGroup shader.
+
 shader_type canvas_item;
 
 void fragment() {

servers/rendering/renderer_rd/renderer_scene_sky_rd.cpp

@@ -855,6 +855,8 @@ void RendererSceneSkyRD::init(RendererStorageRD *p_storage) {
 		storage->shader_initialize(sky_shader.default_shader);
 
 		storage->shader_set_code(sky_shader.default_shader, R"(
+// Default sky shader.
+
 shader_type sky;
 
 void sky() {
@@ -942,6 +944,8 @@ void sky() {
 		storage->shader_initialize(sky_scene_state.fog_shader);
 
 		storage->shader_set_code(sky_scene_state.fog_shader, R"(
+// Default clear color sky shader.
+
 shader_type sky;
 
 uniform vec4 clear_color;

servers/rendering/renderer_rd/renderer_storage_rd.cpp

@@ -9398,6 +9398,8 @@ RendererStorageRD::RendererStorageRD() {
 		particles_shader.default_shader = shader_allocate();
 		shader_initialize(particles_shader.default_shader);
 		shader_set_code(particles_shader.default_shader, R"(
+// Default particles shader.
+
 shader_type particles;
 
 void process() {