Expose SDL GL support functions

Exposes `SDL_GL_SetAttribute`, `SDL_GL_GetAttribute` and `SDL_GL_SwapBuffers`.

`get_attribute` segfaults on my system. I haven't been able to figure out why, after a lot of debugging and experimenting. As far as I can tell it looks like similar code. I figure one of you fine folks will understand the problem, but if not we could just not include it - the function itself is almost useless because it only queries SDL's internal state, not the underlying GL state. Even SDL's own documentation says not to bother with it.