Camera projections are not working in Hydra material networks

[sirpalee]

Describe the bug At the moment connecting camera nodes to UsdShade (to use for camera projection) nodes is not working.

We face several challenges:

• We need to be able to access the Arnold camera node pointer when translating shader networks.
• We have to deal with the multi-threaded sync nature of Hydra, so translating shaders needs to happen after translating cameras.
• Deleting cameras need to update shader networks where the camera pointer is used.