Blender Git Loki

ShapeKey: Refactor a bit `BKE_key_convert_from_...`

Thing is, those functions always reallocate the whole keyblock's data mem,
while in some cases we already have right amount of elements, so we can just
copy over. Further more, `BKE_key_convert_from_offset`, despite its name,
was not making any check nor allocation on keyblock's data elements!

So split 'copy' operation itself in `BKE_key_update_from_...`,
where no mem checks/operations are performed (only an assert).

Only useful in sculpt mode currently, but will be used by fix for T35170 too.