* Ranges between full brightness to half-brightness (50% of fullbright effect).
* Has both software and GL support (I think - GL needs to be doublechecked, but I think it's just GL being murky in general).
* Used by Rings.
* Backport FF_GLOBALANIM and FF_RANDOMANIM extensions to FF_ANIMATE from 2.2.
* Rings use the former instead of their own hacky solution to the problem.
* Also backport FF_VERTICALFLIP, because why not.
* Inverts sprite relative to gravity. I remember wanting this for the flipped death sprite for Jawz, but then Oni drew his own frame for that, so it doesn't matter.
* Refactor vissprite struct a tad, given I did so in internal where I pulled most of these improvements from.
* Should be no in-game affect; let me know if you notice any regressions.
