Support the Hydra point instancer

[sirpalee]

Support the Hydra point instancer for primitives.

Initial support was already added for instancing meshes see render_delegate/instancer.cpp and related files.

A few features are still missing:

• add support for instancing volumes
• calculating transformation motion blur for instance matrices

We can use hdEmbree/instancer.cpp as an example implementation.

[nrusch]

As part of the improvements to instancing, I think the render delegate needs to treat the Rprim that is being instanced as if it is not part of the scene, instead of rendering it. This would match the behavior of Embree and Prman delegates.

Right now, if I create an instancer with 8 instances and a single mesh prototype, I get the 8 instances plus the original prototype in the render.

This may mean continuing to translate the prototype but making it invisible in Arnold, or generating numInstances - 1 ginstance nodes and using the prototype for the last one (with the transform of the final instance) The second approach seems nicer and more Arnold-like, but would require the prototype's transform to be factored into the computation of the instance matrices as well.

[sirpalee]

@nrusch found an issue with updating transformation matrices for the point instancer and created a solution in #69