Blender Git Loki

Multires: Correct averaging

Before that only normal component was averaged, which is not
really correct.

Unfortunately, the new code is somewhat slower due to more
involved math to deal properly with non-quad faces, but the
plan is to move averaging from runtime to edit time, This
means, that mdisps will always be continuous around the edges
and no averaging on every frame change of animated character
will be needed.