USD MeshLight I/O

[ericmehl]

This adds reading and writing of USD mesh lights. A mesh becomes a mesh light in USD by attaching the MeshLightAPI to it. USD itself makes reading mesh lights easy by taking on the addition of the relevant attributes when a mesh has a MeshLightAPI attached to it.
Writing mesh lights is also pretty straightforward - we need to intercept a light with a MeshLight shader, attach the MeshLightAPI to the prim and continue on with writing the parameters.

Checklist

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have tested my change(s) in the test suite, and added new test cases where necessary.
• My code follows the Cortex project's prevailing coding style and conventions.

[johnhaddon]

Thanks Eric! LGTM. If you could update Changes.md at the same time as adding the parameter value test I requested inline, that would be grand. Then we can get a release out with this and my PointInstancer stuff in it...

[johnhaddon]

Would be good to include a few parameter values here, and check they've been written.

[ericmehl]

I added a few parameters with different types and went ahead and squashed it into the original commit : ca1f6e5

[ericmehl]

Thanks John! I added some parameters to the write test and updated the changelog. Both are squashed down to the original commit.

Testing went fine locally - hopefully CI agrees.

[johnhaddon]

Nice one, thanks Eric! CI was happy so I've merged...