Blender Git Loki

Modifier: add support for vertex groups

Allow blending the imported cache with the modifiers stack above the
MeshCache modifier.

This is particularly useful for instance when dealing with cloth
simulations performed in another software, where some parts of the cloth
are completely pinned (non-simulated, following the armature). Indeed,
this would allow modifying the animation in some areas without having to
rebake the other parts or the cloth, resulting in a much more flexible
workflow.

Reviewed By: #modeling, campbellbarton, mont29

Ref D9898