UnityGLTF Shader Variants

[andybak]

Ensures the neccesary UnityGLTF shader variants are included in the build.

Background info: KhronosGroup/UnityGLTF#736

[mikeage]

In general, if you're going to be opening a PR, don't use [CI BUILD] in the commit; it just causes it to build twice (once with (push) and once with (pull_request))

[mikeage]

I don't know what this does, but sure, why not :-)

Note that you should delete the Library cache (no need to delete the Package or Python caches) before merging, as I see this triggers more shader compilations at build time.