f1d4c35f4d
GreasePencil v3 requires some additional processing on its data before writing to blendfile, and after reading from it. Extract this from generic `read/write` code, and put it into dedicated callbacks in Cpp classes that need it (layer groups and leaves currently). NOTE: Work done while exploring possibility to use 'memfile' read/write code for GP editmode undo (!117072), but ended up taking a different approach for it, so this PR is purely a 'no expected changes in behavior' refactor at this point. Pull Request: https://projects.blender.org/blender/blender/pulls/117413