Blender Git Loki

Subdiv: Add ptex offsets to a subdiv structure

This is something what we need to know quite often from various places.
Added it as a cached value in Subdiv itself, so it can be queried easily
from any area.

Shouldn't be a problem from memory usage point of view, it's 4MB per
1M faces coarse mesh. This is very low percentage of mesh itself, and
even lower percentage of highres subdivided mesh.