## OpenXR Controller Model Rendering

Add controller model rendering for some of the tested interaction
profiles (currently only HTC Vive and Valve Index controllers are
included).

OpenXR does not yet provide any api for rendering controller models
in the same way that OpenVR does, though work towards that goal has
been in progress for a long time.  Until that materializes, this change
provides a way to render glTF controller models by using the currently
active interaction profile to select from a few models added to the
repository to be rendered in place of the current green pyramids.
