Keep track of whether textures have alpha values or not

Could skip the alpha test already in the REPLACE mode, but it'd be more interesting with vertex info.

Anyway, this makes it do slightly less work in the case of a full alpha texture with RGBA texfunc.  This isn't super common, though (maybe 3%, though some games > 20% iirc.)  It's an initial step to having more context to disable the alphatest, though.

-[Unknown]