Blender Git Loki

Part-Bugfix, Part-Feature Completion: 'Armature' Option for Mask Modifier
finally works

This commit finally hooks up the Mask Modifier's "Armature" option with the
relevant depsgraph updates on bone selection. Hence, this feature finally works
as it was originally intended - that is, bone selections can be used to control
which parts of the mesh that the mask modifier is applied to are displayed,
giving riggers more freedom to experiment with rigs that don't necessarily
feature overbearing/cluttering widgets.

Regarding the implementation ("has_viz_deps" flag):
This feature is just the "tip of the iceberg" of a number of related set of
rigging/visual animation tools I've had in mind for a while now (dating back to
the introduction of this modifier). Key considerations
- Not all rigs will use this, so we don't want an extra (depsgraph-flush +
search) recalc cost for those that don't use this.
- There are some planned features which will also use this