Blender Git Loki

Today I ran outside screaming and an hour later came back in and made BMLog
reference counted. This fixes various undo bugs caused by dyntopo
needing to execute an undo push but not being able too (e.g. during
file load, and I think it also happens sometimes with global undo).

A much better way of fixing this would be to add unique IDs to mesh
verts and faces, perhaps as a customdata layer.

The root problem is that BMLog assigns unique IDs to mesh elements,
which it does via a series of GHash's. I imagine this is a fairly
serious violation of the undo system's design rules, since it means
BMLogs are tied to specific instances of the bmesh in SculptSession->bm.

There were some hacks to try and get around this, but they were buggy
and needed to push the unstack stack to work, which wasn't possible in
all cases (e.g. if G_MAIN->wm.first->undo_stack is NULL, as it is during
file loading and apparently global undo).

Anyway, long story short each chain of SculptUndoNodes needs some way
to reconstruct their ID GHash's when exiting/entering the chain. The
only way I could think to do this was to make BMLog reference counted,
with BMLogEntry the users.

Like I said, having a proper system to assign unique IDs to mesh
elements would be *much* better.