SDL_LockTexture returns the pitch of the locked texture, so rust-sdl2 should so

In my application I need the pitch of the texture after I locked it, so this (or some similar) modification is required to be compatible with the original SDL 2 API. 

The second commit contains a little bit cleaner implementation, because it does not expose the ugly tuple type, and, what is more important, it makes impossible to forget unlocking the locked texture.
As unsafe_lock is private, and there's no way to lock outside of your lock function, should this become private, too?