Upgrading Houdini From USD 21.05 to 21.08

For the Houdini 19 release, SideFX updated our development baseline to build against USD 21.08 from 21.05, which was used for the early part of the Houdini 19 beta. What follows is a description of that process, from a developer perspective.

An important note here is that we decided to stick with the Ar1.0 library, rather than moving to Ar2.0. This no doubt had a large impact on the size of this upgrade effort.

This upgrade was even easier than the 21.02 to 21.05 upgrade. The only code change that was made to the Houdini baseline as a result of this upgrade was to update a call to the deprecated UsdPrim::GetMaster() function to UsdPrim::GetPrototype() (since the former function has now been removed, rather than simply deprecated).