Fix #34216: uv unwrap with some faces pinned to point the normals down will now
automatically flip the unpinned faces to point down too, instead of trying to
unwrap them in the other direction and giving bad results.

If there's a mix of faces pinned up and down it will pick the direction with the
biggest area.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54525 954f8c5b-7b00-dc11-b283-0030488c597c