optimize single GeomSubset

[drwave]

Because USD's Hydra's StreamGL renderer does not currently support GeomSubset, you can trivially exported textured geometry from this exporter that current usdview w/Hydra doesn't render correctly (i.e. no texture). In many cases, there are single faces (with two sides) that are using GeomSubset, but it's really not a subset, it's the whole thing.

In that case, we should omit doing the material as a subset and just apply the material to the whole mesh.