Blender Git Loki

Multires: Refactor propagation to the higher levels

Now it is forumlated in terms of deltas, and consists of the
following steps:

- Original displacement at the reshape level are being backed up.
- New displacement is being written by the reshape routines.
- Delta of the displacement is calculated.
- Deltas are propagated to the higher levels, which also includes
their interpolation/
- Original displacement is restored.
- New interpolated displacements are added to the original ones.

This is a base ground for the upcoming change related on using
Catmull-Clark smoothing for the deltas instead of linear
interpolation. Currently is no changes for artists, just preparing
for upcoming work.