When in material display mode, mesh_calc_modifiers() calculates the
orco DerivedMesh, which uses a different CustomDataMask. In
particular, it does not necessarily include the current modifier's
requiredDataMask, so those layers might get set to NO_COPY. For the
skin modifier, this resulted in a crash when the modifier internally
copies the DerivedMesh and the output does not contain the expected
MVertSkin layer.
Fixed by adding the requiredDataMask to the orco DM's CustomDataMask.
Also added a debugging function to customdata.c:
customData_mask_layers__print(CustomDataMask mask);
This will print out the names of all the CD layer types in the mask.
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