Support for panoramic cameras

[Vochsel]

Basic support is added in this branch: https://github.com/tangent-opensource/hdcycles/tree/feature/panoramic_camera

Few missing features before PR ready.

We rely on usdcycles schema for knowing if a camera should be panoramic. Ideally we would override the UsdGeomCamera projection attribute, however this seems harder to override in Houdini and should really be extended via USD schema plugin. We could do this in usdCycles, however would require some rnd.