Split remaining graphics files on a per-API basis

Another API file split as in #2243 covering the major areas of Graphics where we have lots of platform variance. As always, additional sets of eyes are appreciated. These involve a lot of tedium and #ifdef juggling, so I'm bound to make mistakes eventually.

(Edited to expand scope of the original PR so as not to split it up among a lot of pull requests. We can cherry-pick and reopen if we need to.)