Deleting material assigned to GeomSubset does not refresh the material in the viewport

[aloysbaillet]

Description of Issue

Deleting a material assigned to a GeomSubset does not repopulate the Hydra prim.
PR with simple fix is coming up shortly.

Steps to Reproduce

1. Download and unzip mat-on-geomsubset.zip
2. usdview mat-on-geomsubset.usda
3. Select /World/Looks/PreviewSurface Material prim
4. Deactivate that /World/Looks/PreviewSurface Material
5. The cylinder mesh stays green, but should become gray

System Information (OS, Hardware)

NA

Package Versions

All USD versions up to 22.05

Build Flags

NA

[jilliene]

Filed as internal issue #USD-7336

[marktucker]

Before I realized exactly what the problem was, I created another test case for what turned out to be this same issue. My case is a little different, because I'm not deleting the material - I'm replaceing one material with another in a way that causes hydra to "repopulate" the material sprim. But the associated PR also fixes this case (thank you @aloysbaillet !). Since I did the work I thought I'd attach the test case in spite of the redundancy.

1. Open grid.usda from this archive in usdview
   material_update.zip
2. Run the following in the interpreter window:
   p = usdviewApi.stage.GetPrimAtPath('/matlib') p.GetReferences().SetReferences([Sdf.Reference('', '/materiallibrary2')])
3. The material should change from red to blue, but it does not.
4. Try the same thing with "gridworks.usda". In this case the update happens. The only difference between grid and gridworks is that in grid the material is bound to the geom subset, and in gridworks the material is bound to the mesh prim.