In the "All" mode, handle vertex group data separately from other
attributes, which allows copying all vertex groups at once. In a file
with 172 vertex groups, the node became about 12 times faster, from
25.8 to 2.2 ms. With fewer vertex groups the change will be smaller.
Theoretically a similar optimization would work elsewhere, but ideally
we would have a more generalized concept of sparsely stored attributes
first. In the meantime this is a simple way to improve some common
rigging use cases.
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