ColorProcessor creation was done for every exported frame and the input
reference space was passed as a role string (e.g. "scene_linear"). If
getProcessor() rejected the role name the worker would crash, the pool
would retry the frame, and export throughput would drop to near zero with
minimal CPU/GPU usage.
- Canonicalize role names to colorspace names in ColorProcessor so
"scene_linear" resolves to the config's actual colorspace.
- Wrap processor creation in a try/catch and initialize cpu_processor_ to
nullptr on failure instead of dereferencing a null processor.
- Cache created ColorProcessor objects in the worker keyed by transform
so OCIO processor/shader setup is only paid once per export.