Modifier previews should be implemented by a more generic system.
The current system is already a hack and needed a lot of work
to get it working again in 2.80 and even so that would be replaced by
another system in the near future.
For Vertex Colors we have a work around in place by using Workbench
Vertex Colors. For Vertex Weights we loose the previewing. Not sure
targetting weight is working (even for 279).
Reviewed By: brecht
Maniphest Tasks: T63857
Differential Revision: https://developer.blender.org/D4734
6d09fa3577