* Leave code for building the render delegate against other applications and
their USD libraries to the Cycles repository, since this is not a great fit.
In the Blender repository, always use Blender's USD libraries now that they
include Hydra support.
* Hide non-USD symbols from the hdCycles shared library, to avoid library
version conflicts.
* Share Apple framework linking between the standalone app and plugin.
* Add cycles_hydra module, to be shared between the standalone app and plugin.
* Bring external libs code in sync with standalone repo, adding various missing
libraries.
* Move some cmake include directories to the top level cycles source folder
because we need to control their global order, to ensure we link against the
correct headers with mixed Blender libraries and external USD libraries.
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