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.
This makes it more generic process to perform actions which
depend on ptex face + (u, v) and on subdivided vertex index.
Currently it is still just a subdivision calculation process,
but same foreach callbacks can easily be used to propagate
displacement from known vertex locations back to displacement
grids.
