Blender Git Loki

Sculpt: More BMLog fixes and cleanups

This commit rewrites much of the core BMLog
serialization logic.

BMLog originally did not support mesh
elements changing their internal topology,
which created ID conflicts in the log. Thus
it was impossible to use much of the BMesh API.
Instead DynTopo had its own implementations of
BM_edge_collapse and triangle edge splitting
that worked by recreating local mesh topology
from scratch.

I got rid of these functions a while ago and
tried to add support for elements changing
topology to BMLog. Unfortunatey I struggled to work
out all of the edge cases, and it turned out easier to
refactor the core serializer methods wholesale.