Additional Bounding Box Display Options

When working with large scenes, it might be required to switch the viewport to BBox display to retain interactive speed. Whilst USD support "drawmodes" which allow some control over this, it would be much more convenient to users if the viewport could handle this in a more dynamic way. A sub-set or combination of the following features would be a great help with this:

*Shaded bounding boxes (by far the most important)
*Bounding boxes for objects smaller than a certain threshold
*Bounding boxes for objects at a certain distance from camera
*Bouding boxes for objects above a poly count threshold
*A clever combination of all of the above which adapts to maintain a "target FPS setting"

The issue names no files, tests, or entry points. Begin by scoping the viewport bounding-box display requirements and defining one specific option with a measurable completion criterion before implementation.

Written by the indexing model from the issue text.