Support instances of mesh lights

Instances of mesh lights aren't currently supported for multiple reasons :
1) in USD we have a single mesh primitive tagged as a light, whereas arnold still uses the "dual node" system with a mesh + a light. At the moment, if we have USD instances of this mesh primitive this ends up with instances of the arnold mesh, and not instances of the light.

2) at the moment arnold ignores the transform of mesh lights, so using an instancer on the lights wouldn't be enough.

The issue does not name files, tests, or entry points. Start by tracing how USD mesh-light instances are represented against Arnold's mesh-plus-light dual-node model, then examine how mesh-light transforms are handled. Done means instances resolve to lights rather than only Arnold meshes and their transforms are honored.

Written by the indexing model from the issue text.