The code that determined the "base slot" (i.e. the buffer object index
for the first morphed attribute) was very fragile and buggy.
MorphHelper now simply looks up the slot index in a stored mapping,
rather than trying to mimic the AssetLoader to determine the base slot.
Until now, most of the models we tested against only morphed the
position attribute, and never had a large set of target weights that
could be non-zero in a given time slice.
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