Blender Git Loki

Fix T58210, Part II: Surface Deform modifier (un)binding is broken.

This fixes/clarifies Surface Deform evaluation code that does the
binding, since that part should only be called outside of depsgraph
evaluation, with orig data-blocks and not CoW ones.

Now we have a decent amount of asserts and checks to ensure eveything
works as expected.

Also had to add a special case to get target's mesh in binding case,
since often target's evaluated mesh is not available, in that case (and
in that case only), we can actually compute that mesh (because we are
out of depsgraph evaluation).