Support subdiv dicing camera

[sebastienblor]

Changes proposed in this pull request
When reading the render settings, I'm now reading the attributes subdiv_dicing_camera, subdiv_frustum_culling and subdiv_frustum_padding and setting the arnold options node accordingly.

The most tricky part is the dicing camera, as we need to point to another AtNode in the scene, that might not exist yet when we're reading it. I'm following a similar system that what is being done for the render camera, or for atmosphere : I'm storing the SdfPath of the dicing cam, and then in RenderPass::_Execute I'm looking for it in the arnold scene based on its name.

Something that wasn't strictly needed in this PR, but that I preferred doing to avoid crashes, is that when we're about to destroy a camera node, we check if it's not referenced in the options, and if so I reset the options attribute

Issues fixed in this pull request
Fixes #1062