DRW / Render: Add support for render pipeline in drawmanager.

For simplicity we choose to execute the rendering of Opengl engines in the main thread and block the interface.
This might be addressed in the future at least for video rendering.

A drawmanager wrapper (DRW_render_to_image) is called by the render pipeline to set up the Opengl state and then call the specific draw_engine->render_to_image function.

source/blender/draw/engines/external/external_engine.c

@@ -218,6 +218,7 @@ DrawEngineType draw_engine_external_type = {
 	&external_draw_scene,
 	NULL,
 	NULL,
+	NULL,
 };
 
 /* Note: currently unused, we should not register unless we want to see this when debugging the view. */