Fix #35011: UV unwrap giving poor results for ngons.

Problem is that the scanfill result gives poorly shaped triangles, which is fine
for OpenGL drawing but does not work well for unwrapping. Another problem was
that it was doing random float offsets to avoid some degenerate cases, but this
meant that unwrapping a part of a mesh or the whole thing could give different
results for a UV island. Now UV unwrap does own triangulation of ngons.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56152 954f8c5b-7b00-dc11-b283-0030488c597c