Support instances of mesh lights

[sebastienblor]

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.

[autodesk-oss-arnold-bot]

Issue synced internally to ARNOLD-16909