Blender Git Loki

Fix T42543 projection painting fails when faces are perpendicular to screen.

This is UV barycentric interpolation failing for triangles where vertices are
colinear in screen space. To fix this, we detect this early on so we do the
interpolation on edges instead of triangles.

Such triangles will never be painted on unless we have occlusion and culling
off (since, arguably they are self occluded and almost back-facing).

The code still does not fill the whole area but this can be checked separately.