Implement video::get_video_info

Also with needed conversion for `SDL_PixelFormat` and `SDL_Palette` to their Rust counterparts.

This is the first Rust code I've written, so please point out anywhere that would be better served by some core function or idiom I haven't seen yet. In particular I think the palette member of PixelFormat should perhaps be an Option<Palette>. You let me know and I'll update things :)