ENGINES: We must poll events in order to show the window on some platforms.

This is a regression from b86840087. The side effect of it that on Mac the
splash screen in not visible at all. I hope that this does not trigger same crash,
as it happens now only once.

engines/engine.cpp

@@ -261,6 +261,8 @@ void splashScreen() {
 	// Delay 0.6 secs
 	uint time0 = g_system->getMillis();
 	Common::Event event;
+	g_system->getEventManager()->pollEvent(event);
+
 	while (time0 + 600 > g_system->getMillis()) {
 		g_system->delayMillis(10);
 	}