Make love.event.clear thread-safe, though it's only callable from the main thread (with SDL backend, anyway)

src/modules/event/sdl/Event.cpp

@@ -77,11 +77,7 @@ namespace sdl
 			// Do nothing with 'e' ...
 		}
 
-		while (!queue.empty())
-		{
-			queue.back()->release();
-			queue.pop();
-		}
+		love::event::Event::clear();
 	}
 
 	Message *Event::convert(SDL_Event & e)