TexCache: Decimate CLUT variants more often

[unknownbrackets]

May help #10418. Though we should probably try to handle going over limit better than crashing.

This cuts down on the first mission's texture allocation to 1/3 (about 120 instead of 360.) Having 6+ variants shouldn't be too common, so should be mostly safe.

This impacts Popolocrois by about 14% reduction in performance with retain changed textures on, but makes it very slightly faster with it off (though still much slower than with it on.) I think that's pretty acceptable, and it's an edge case game.

-[Unknown]

[hrydgard]

Bit worried that the extra lookups in the texture cache map could cost something in games that are unaffected by the problem, but I guess usually it'll already be in cache..

First thought about nicer ways to handle running out, I guess we could have a little list or flagging of textures that are "in-flight", and instakill those that aren't if we run out of memory... we'll soon have implemented a GL driver going too far down that path though ;)

[hrydgard]

Couldn't measure a consistent difference in GTA: LCS, which is a game that uses many textures per frame (>1000). So not really worried about slowdown, let's try this.