Decimate in clut stuff, adjustments to IsReallyAClear()

Even on desktop, the clear stuff provides a small performance increase in for example Gods Eater Burst.  There's a comment that the depth is wrong... but I'm not sure I see why it would be?

I noticed that we leave vertex arrays enabled even when calling DrawActiveTexture, etc.  I guess this is probably okay, but is it a problem we disable them in those funcs?

-[Unknown]