I didn't think it was worth it, and then I found myself needing to
update a dozen of them. The code isn't especially pretty, but it works,
so that's fine.
Also fixes several issues with us using the wrong texture (closes#572).
I've put together a wiki page[1] which describes each texture in a
little more detail.
[1] https://github.com/SquidDev-CC/CC-Tweaked/wiki/Monitor-texture-reference