Add support for "holdout" geometry

[pmolodo]

Animators often use tracked stand-in geo which is used to make the plate show up "on top of" animated rigs. With normal maya, they can set this stand in geo to be a "holdout", and it will show the plate anywhere that geo is.

Using a standard AL_USDMaya proxy shape, however, the geo rendered by hydra does not "respect" the holdout, and will show up on top of the plate, even though the hold-out geo is closer. Presumably this is because they are from hydra + VP2, and don't know how to coordinate properly.

However, if we could implement this in maya-to-hydra, then they'd be able to have both maya + usd geo rendered in the same place, and we could get it to work.