Vertex preview (working for 2d, broken but sometimes works for 3d)

I tried to copy the algo from softgpu, and it has broken 3d also, so may be related... but I'm probably doing something wrong.  Also, thinking, not sure if I want z to always be a fixed value maybe...

Well, I think the 2d may be broken in some cases too (probably still need to apply offset or something?)

But at least it works.  You can see which parts of the texture are being used and etc.

-[Unknown]