According to SDL's docs, the window surface is freed automatically by
other functions (SDL_Quit in SDL1.2, and SDL_DestroyWindow in SDL2).
I think this might have actually been the cause of those segmentation
errors upon closing that I noticed back when I opened #114.
On my PC, the SDL2 build doesn't 'obtain' the S16 format it
requests, but since the rest of the SDL2 frontend expects S16, this
causes the audio to fail. So, instead, I force SDL to accept S16, and
just convert to the native format implicitly.
I also applied this to every sound format setting, since I imagine the
frontend doesn't support obtaining only one channel, or a non 48kHz
frequency.
