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Surface Deform modifier: add vertex group and strength control.

This commit aims to add functionality to the surface deform modifier that
gives more control and allows it to work better with the modifier stack.
* Maintains compatibility with older files. The default settings keep it
so that the whole object is bound and vertex coordinates get overwritten
as the modifier currently does.
* Turns the deformations from an absolute vertex coordinate overwrite into
an additive offset from the vertex location before the modifier to the
resulting bound deformation. This gives the ability to control the
strength of the deformation and mix the deformation of the modifier
with the modifier stack that comes before it.
* Also adds in a vertex group with the invert option. This is applied after
the bind deformation is added. So the whole object is still bound to target,
and the vertex group filters afterwards what parts get affected.
I experimented with a version to only binds the geometry weighted to the
vertex group, but that would break compatibility with old files.
I may bring it in later as a separate option/mode for the surface deform.

With several fixes from @mont29.

Reviewed By: mont29

Differencial Revision: https://developer.blender.org/D6894