Crashes/cleanup in softgpu, fix a couple warnings, remove some unused gstate_c

Minor changes mostly, but now the softgpu crashes a bit less... also some typesafety.

zMin and zMax weren't even used under the precomputed gstate_c, so I just removed them.

Also added an alternate ReadPosZ16() to skip the throughmode z scaling to 1.0f.  This fixes some tests I'm trying to write to test stencil/depth.

-[Unknown]