Implement some basic vertex previews on prim.

3D doesn't work correctly (sometimes it does...)  2D should be working.
Clean up no-inds rectange preview.
Clean up Mat3x3 etc. constness.
Fix Vec3ByMatrix44() and use it for matrix math.
Fix 3d vertex preview.
Avoid some operator overloads.

Causing ambiguity.
Cleanup, don't even need z here.
Fix vertex preview when framebuf is an odd size.

The previous stuff was wrong, just happened to work.
Merge pull request #4567 from unknownbrackets/debugger

Vertex preview (should work for 2D and 3D now)