This allows to mix between the result of 2 modifiers, with both
using the same input state. This is useful for having a mesh deform
and armature deform working together.
However! This functionality could have been presented better...
this is actually Node editor stuff!
Now it works by adding a "MM" button, next to the "overall vgroup"
option. If MM is pressed, the input of this modifier is the same as
the input of the previous modifier.
Only the armature modifier has this option now...
7accd3a662