Blender Git Loki

Depsgraph: First draft of graph filtering API implementation

When this works correctly, we should be able to feed in an existing
depsgraph instance, and get out a "filtered" copy of it that contains
only the subset of nodes needed to evaluate what we're interested in.

The current implementation only filters on ID blocks/nodes,
and starts by building a full new depsgraph instance first.
I'd originally intended to do it per operation instead, copying
over individual nodes as appropriate to have the smallest and least
memory intensive graph possible. However, I ended up running into
into problems with function binding + COW arguments, hence the
current slow solution.