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.

editor/plugins/texture_3d_editor_plugin.cpp

@@ -79,6 +79,8 @@ void Texture3DEditor::_update_material() {
 void Texture3DEditor::_make_shaders() {
 	shader.instantiate();
 	shader->set_code(R"(
+// Texture3DEditor preview shader.
+
 shader_type canvas_item;
 
 uniform sampler3D tex;