Try and prevent premature fullscreen exits

* Sometimes window events are received prematurely (probably caused by the initial window resizing)
* Ignore window events until the window is actually visible

src/framework/WindowManager.cpp

@@ -276,11 +276,14 @@ void WindowManager::eventLoop()
 				(event.type == SDL_MOUSEMOTION || event.type == SDL_MOUSEBUTTONDOWN ||
 				 event.type == SDL_KEYDOWN)) {
 
+			// don't handle events prematurely
+			if((SDL_GetWindowFlags(m_Window) & SDL_WINDOW_SHOWN)) {
 				// we're in screensaver mode so exit on user input
 				SDL_GL_DeleteContext(m_GLContext);
 				SDL_DestroyWindow(m_Window);
 				SDL_Quit();
 			}
+		}
 		else if (event.motion.state & (SDL_BUTTON(1) | SDL_BUTTON(3)) &&
 				 event.type == SDL_MOUSEMOTION) {