SDL: Call multiplayer->update() from the user code thread

The multiplayer update scans for messages and passes them in to the
user defined function on_message(). This should be done in the user
code thread for two reasons:

1. Calling it from a different thread means on_message() and update()
are being run in different threads, and therefore all code in both of
them has to be thread safe. This is pretty hard to do with the 32blit
API and not really in the spirit of what this is supposed to be.

2. on_message() is a user-supplied function and so bugs can make it
hang. We can detect hangs in update() and render() thanks to running
them on the user thread (via blit::tick()). Running on_message() on
that thread allows to detect hangs there too.

32blit-sdl/Main.cpp

@@ -105,7 +105,6 @@ void handle_event(SDL_Event &event) {
 
 		default:
 			if(event.type == System::loop_event) {
-				blit_multiplayer->update();
 				blit_renderer->update(blit_system);
 				blit_system->notify_redraw();
 				blit_renderer->present();

32blit-sdl/System.cpp

@@ -269,6 +269,7 @@ void System::loop()
 	blit::joystick.y = shadow_joystick[1];
 	SDL_UnlockMutex(m_input);
 	blit::tick(::now());
+    blit_multiplayer->update();
 }
 
 Uint32 System::mode() {