Add a weight multiplier option for all operations.

[chris-lesage]

Add a weight multiplier. So if you set it to 1.0 it will act as normal. If you set it to -1.0 you'll get the opposite effect. If you set it to 0.25, you'll get a quarter of the effect.

This should definitely work with Vertex Smash.

For Blendshape Splits, I'll have to consider what a weight would mean. Should the multiplier be relative to the neutral pose?