USDScene : Fix loading of animated materials and lights

[johnhaddon]

No description provided.

[johnhaddon]

I took advantage of IECoreUSD technically (but ridiculously) still being in contrib to just change the function signature here. I can add an overload if we think maintaining ABI is important to anyone.

[danieldresser-ie]

Looks good to me. The only question I'm left with after reading through it is why shaderNetworkMightBeTimeVarying needs a cache, but lightMightBeTimeVarying doesn't - there's probably a reasonable explanation though, perhaps just because light networks are generally expected to be so much smaller than other shader networks. Regardless, this is just a performance question, and doesn't affect correctness, so I don't think it should hold up getting this merged.

[johnhaddon]

The only question I'm left with after reading through it is why shaderNetworkMightBeTimeVarying needs a cache, but lightMightBeTimeVarying doesn't

For the same reason that we were already caching calls to readShaderNetwork() but not to readLight(). Partly, as you say, because materials are typically much more complex than lights (which in many cases have no additional shaders). But also because there are far more opportunities for cache hits - it's very common for many prims to be bound to the same material, so we commonly need to query/load the same material many many times. The only way caching would make sense for lights would be if the same light was instanced many times - that's not an impossibility, but our assumption so far has been that it's not important for our use cases. It may be worth visiting that again at some point, but I judged it to be out of scope for this PR.