Use the mechanism where there is one reference directory, and an override
directory for the cases that are known to be different.
The override is kept only where the committed USD reference and Hydra
reference differ beyond the test threshold; otherwise the Hydra render
is redundant and shares the USD reference. This helps work towards
making them match, and reduces the repository checkout size.
Pull Request: https://projects.blender.org/blender/blender/pulls/160433
