Do not wait for programs main() to exit after the SDL drawing function

returns... this means that the user has closed the program, so we can't
very well wait for the program to exit normally can we?

src/xpdev/sdlfuncs.c

@@ -535,7 +535,8 @@ int SDL_main_env(int argc, char **argv, char **env)
 		sdl.SemWait(sdl_main_sem);
 		if(sdl_drawing_thread!=NULL)
 			sdl_drawing_thread(NULL);
-		sdl.SemWait(sdl_exit_sem);
+		else
+			sdl.SemWait(sdl_exit_sem);
 	}
 	else
 		main_ret=XPDEV_main(argc, argv, env);