The documentation on SDL_RenderPresent function states that
"the backbuffer should be considered invalidated after each present",
so copy the entire texture on each redraw.

On the other hand, SDL_UpdateTexture function is described as
"fairly slow function", so restrict it to just the changed pixels.

Also added SDL_RenderClear call, as suggested in the documentation
page on SDL_RenderPresent.

Signed-off-by: Anatoly Trosinenko <anatoly.trosinenko@gmail.com>
Message-id: 20180205133228.25082-1-anatoly.trosinenko@gmail.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>