Added draw_raw_atlas_texture

[jackolantern]

I'm not sure what the dev process is for this crate, and whether random PRs are okay. I don't think this there is an issue of feature request for this change, but this is something that I very much needed and so I added it an would like to get it upstream.

The change allows - for example - a button to have a graphical label that uses a part of a Texture instead of an Image.

[not-fl3]

Thanks for PR! Submitting a PR is always OK, no worries about it!

I did not really looked at it yet, but, maybe, it is possible to add a "source" field to the normal DrawTexture, somewhat similar to the one from draw_texture_ex? It feels more natural, but I did not dived into it yet and it might not be possible.