When retrieving a color attribute to paint it, the layer data needs to
be unshared so that it doesn't modify data from separate meshes.
In the past I think we've unknowingly avoided this problem by porting
code to the new attribute API. I've been refactoring `PBVH` and
`SculptSession` to make that possible (removing the long-lived
layer pointers in the two structs). But that wouldn't fit as a bug fix.
In the meantime, make sure the color attribute data is un-shared
and separate "for read" and "for write" versions of the function.
Pull Request: https://projects.blender.org/blender/blender/pulls/114119
692292536f