This revision moves the vertex color reading and writing in the USD import and export functions over to the new Mesh Attributes API. I have removed anything else (new features or unnecessary changes) that was present in the prior patches to focus only on this task.
On the import side, I've introduced a class method named read_custom_data. In this function is the call-out for reading mesh colors. As requested, this function is intended to be the starting point for future Attribute reads, with methods like the new read_color_data* methods being called when a USD primvar matches a specific heuristic. UVs will (in the future, not in this revision) also need to be processed here. In a later patch, any primvars that do not match a heuristic can be imported as generic Attributes. There is a matching function on the export side, write_custom_data.
Attached is a .blend file for testing. The plane has five Color Attributes. The colors should be visibly the same when exported and re-imported.
I have also enabled color attribute imports by default. I believe it would be counter intuitive for most users for this feature to be off-- it means that at some point, a person round-tripping with default settings will lose data.
Pull Request: https://projects.blender.org/blender/blender/pulls/105347
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