[Hydra] Changing primvars always causes a full prim resync

[marktucker]

Description of Issue

Changing a primvar in any way (adding or removing a primvar, changing a primvar value, changing primvar interpolation, changing a primvar value or interpolation of an inherited primvar, etc) all cause a full hydra resync of the prim and all its children (because of possible primvar inheritance). Instead, a simple DirtyPrimvar change should be enough to resync the prim(s).

This change will likely cause problems for some render delegates that currently rely on a full resync when primvars are added or removed (and note the persistent nature of the primvar descriptors in the value cache - there is no way to remove a primvar from the desriptors stored in the cache). I would prefer if it were up to the render delegate to handle the adding/removal of a primvar without a full resync, even if this did require updating some delegates...

Steps to Reproduce

1. Set TF_DEBUG=USDIMAGING_CHANGES
2. Load the attached file into usdview:
   sphere.zip
3. In the interpreter run the following code to change a primvar value:
   a = usdviewApi.stage.GetObjectAtPath("/geo.primvars:foo")
   a.Set(1.0)
4. Notice the messages about repopulating /geo/sphere/mesh_0. Ideally this would just cause a DirtyPrimvar change notification sent to the Sync function for the rprim. See the discussion on https://groups.google.com/forum/#!topic/usd-interest/vdCorI2iFuI

Package Versions

USD 19.11

[jilliene]

Filed as internal issue #USD-5775