Add buf(), buf_mut() and into_buf()

[yoanlcq]

Hello,

This adds accessors to Img's buf member and deprecates direct access to it.

I "need" this because I'm using ImgRef as a container to transfer image data to OpenGL, which requires a pointer to the pixels.
This cannot be done efficiently with iterators (technically it can, but it's hacky), and I don't feel comfortable about directly using the buf member knowing it will be private someday.
In short, it feels like Img "steals" the container from me, and I'd like it back :)

Thanks in advance :) I acknowledge that this is probably a bit petty and the newly introduced deprecated warnings may annoy a bunch of people.

[kornelski]

Thank you