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/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() {