Update to SDL2 snapshot 2013-08-29 (rev 7712).
Update to SDL_mixer snapshot 2013-08-24 (rev 659).
Update to SDL_image snapshot 2013-08-12 (rev 422).
Update to SDL_net snapshot 2013-08-12 (rev 251).
Remove obsolete docs.patch.
Update to SDL_ttf snapshot 2013-08-12 (rev 255).