Old bug fix: Uv smart stitch failed to detect angle if islands were
rotated close to 180 degrees and there were edges both below and above
the 180 degree threshhold. Separating and averaging the negative and
positive angles seems to solve the issue making the tool a bit more
robust ;)

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54478 954f8c5b-7b00-dc11-b283-0030488c597c