* Fix for #31581. The issue was that we scaled the face prior to
projecting it. The original paper suggests to simply interpolate between
the two points of an edge if the distance of the point to that edge is
smaller than a threshold.

* Fixed both 3D and 2D code to utilize this. Possibly other places in
blender where this scaling is done will have to be adjusted.

* Changed vertex interpolation to use 2D interpolation, since it already
did projection on plane and 2d calculations are faster.

* Also added notifier on hard recalc when uvcalc_transfor_correction is
used. Results in instant feedback on UV editor when edge sliding.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53641 954f8c5b-7b00-dc11-b283-0030488c597c