Can't select individual instances in viewport

[murphyeoin]

Logging this for visiblity, it's been mentioned by a few people both internally and externally, but no Issue logged so far. When selecting instances in the Hydra viewport, there are 2 problems:

1. Querying the selected paths always results in the first instance being returned.
2. Selection highlighting selects all instances. This is a direct result of https://github.com/PixarAnimationStudios/USD/issues/444. We can attempt to implement a suggested masking workaround see https://groups.google.com/forum/#!topic/usd-interest/3L2I93UjHss

We have a fix for 1. internally which will be released soon, 2 is still WIP