Add ability to call code on rendering thread

As more users use compute in Godot 4, the way they do is most likely incompatible when running
on separate threads and will start erroring soon as we improve the thread safety of the render thread.

To properly run code on the render thread, this function was added. Use like this:

```GDScript

func initialize_compute_code():
	....

func update_compute_code(custom_data):
	...

func _ready():
	RenderingServer.call_on_render_thread( initialize_compute_code )

func _process():
	RenderingServer.call_on_render_thread( update_compute_code.bind(with_data) )

```

servers/rendering/rendering_server_default.cpp

@@ -386,6 +386,12 @@ void RenderingServerDefault::draw(bool p_swap_buffers, double frame_step) {
 	}
 }
 
+void RenderingServerDefault::_call_on_render_thread(const Callable &p_callable) {
+	Variant ret;
+	Callable::CallError ce;
+	p_callable.callp(nullptr, 0, ret, ce);
+}
+
 RenderingServerDefault::RenderingServerDefault(bool p_create_thread) :
 		command_queue(p_create_thread) {
 	RenderingServer::init();