Polish interaction between windowing, input and rendering

- Adapt GL make/release API to the current architecture.
- Fix DisplayServer being locked while dispatching input (prevent deadlocks).

servers/physics_server_2d_wrap_mt.cpp

@@ -96,7 +96,6 @@ void PhysicsServer2DWrapMT::end_sync() {
 
 void PhysicsServer2DWrapMT::init() {
 	if (create_thread) {
-		//OS::get_singleton()->release_rendering_thread();
 		thread.start(_thread_callback, this);
 		while (!step_thread_up.is_set()) {
 			OS::get_singleton()->delay_usec(1000);