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. 

I use it in the following way: 
```rust
let (c_buffer, pitch) = match (screen_texture.lock(None)) {
        Ok( (c_buffer, pitch) ) => {
            (c_buffer, pitch)
        },
        Err(e) =>  fail!(e)
    };
```