* Compress all test blend files to save space. Note these files were not
resaved in Blender, but compressed directly with zstd so that any
versioning of old files is still tested.
* Compress some heavy EXR files with DWAA
* Merge blend files testing same BSDF with different parameters.
* Reduce resolution or object size for some slow tests.
* Move volume grid related files from volume to openvdb folder.
* Remove some unused and obsolete opengl test files
* Remove old EEVEE reference images
.blend files on disk: 977 MB -> 224 MB
Test data repo on disk: 1343 MB -> 449 MB
Cycles test time: 163s -> 129s
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