sdl2: Work around SDL2 SDL_ShowWindow() bug

Apparently it is possible for X to send an event to a hidden SDL2
window, leading to SDL2 believing it is now shown. SDL2 will pass the
SDL_WINDOWEVENT_SHOWN message to the application without actually
showing the window; the problem is that the next SDL_ShowWindow() will
be a no-op because SDL2 assumes the window is already shown.

The correct way to react to SDL_WINDOWEVENT_SHOWN would be to clear
scon->hidden (analogous for SDL_WINDOWEVENT_HIDDEN). However, due to the
window not actually being shown, this will somehow not be correct after
all.

Therefore, just hide the window on SDL_WINDOWEVENT_SHOWN if it is
supposed to be hidden (and analogous for SDL_WINDOWEVENT_HIDDEN).

Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>