pxr::UsdMtlxGetPackedUsdValues does not work for vector types

[spitzak]

Description of Issue

Vector MX types have commas in their string representations and pxr::UsdMtlxGetPackedUsdValues assumes they don't.

The MX code appears to just assume there are N-1 commas inside the strings for vectors of size N, so it is probably best to replicate that. It also looks like it would work to split the string at ", " (comma + space) rather than at all commas.

It might be nice to make the _GetUsdValue(string, type) function in pxr/usd/usdMtlx/utils.cpp public as access to that would make working around this much easier.

Steps to Reproduce

Try to get the enumeration list for the ND_luminance_color3 field of a MX Luminance shader.

[jesschimein]

Filed as internal issue #USD-10451