Blender Git Loki

Depsgraph: Forbid flush from copy-on-write operations down the road

Old behavior: tagging ID with DEG_TAG_COPY_ON_WRITE will do copy-on-write
(as requested), but will also flush changes to all operations with depends
on it. This means, for example, tagging object for copy-on-write will force
its modifier stack to be evaluated.

This was needed in the earlier days of copy-on-write when things were not
well defined and when lots of areas were not doing proper tagging.

New behavior: tagging ID with DEG_TAG_COPY_ON_WRITE will only ensure copy
of the dadatblock is up to date, without flushing updates to any dependencies.

This means following things:

- We can update parts of ID without invoking heavy computation of other
parts of same ID. For example, tagging object for COPY_ON_WRITE update
on mode change will not force modifiers stack to update.

- If some dependent datablock is dependent on pointers which are invalidated
by copy-on-write (like, evaluated mesh referencing custom data layers from
original mesh), this will either require explicit tag or explicit relation
in the dependency graph.

Currently can not find a faulty case since tagging of mesh happens with
either 0 (which means, everything) or with GEOMETRY, which also forces
all dependent modifier stacks to be re-evaluated.

This fixes missing PBVH when going into sculpt mode (sculpt mode toggle
was tagging object for COPY_ON_WRITE update, which was forcing modifier
stack to be updated, which was freeing PBVH.

Some other operations might also become faster with this change.