Turn the system mouse cursor back on before VideoQuit().

This is good policy, so it doesn't have a chance to leave it hidden on targets
 that wouldn't necessarily reset it by default, but it also fixes a crash if
 you try to use a message box after SDL_Quit() is called.

Fixes Bugzilla #1969.

src/video/SDL_video.c

@@ -2247,6 +2247,8 @@ SDL_VideoQuit(void)
 
     SDL_EnableScreenSaver();
 
+    SDL_ShowCursor(1);
+
     /* Clean up the system video */
     while (_this->windows) {
         SDL_DestroyWindow(_this->windows);