Originally caused by 6514bb05ea
More cases where the active/default color attributes were not set
correctly:
[1] Using the old Python vertex_colors API (vertex_colors.new)
[2] OBJ importer
[3] Collada importer
[4] Data Transfer layout (both standalone operator and "Generate Data
Layers" from the modifier)
Similar to 101d04f41f.
Brought over from https://archive.blender.org/developer/D16977, see
discussion there why some of the code for data transfer is not for the
genereal attribute API.