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.
This might be better rendered as 'with_lock' or something