Use a separate index for indexing into `Array<TransDataVertSlideVert>
r_sv`, as that only contains the selected items (and not all items
considered for proportional editing).
Since the introduction of the sorted index map (df6d345bb4) the
selected "transform items" are simply visited first, but not sorted to
the front of the array. This means that these array indices cannot be
used to index into other arrays that only contain sorted data, because
they will overflow.
Ref !140976
dff44f1413