Improve zfreeze in software renderer.

Previous code only worked if the reference zplane was flat. In other cases it used the position of vertex one each new polygon to offset the lookup into the zplane.

NHL 2003 shadows now render without zfighting, but most other games with zfreeze have issues. 
I'd go for a const/define here like the old code did, possibly with some additional commenting on why it should be "a little less than 0.5".