Avoid modifying the order of transform system data. Instead, create an
index map and use that to traverse the data arrays in sorted order.
The issue observed in #139042 stems from the assumption, in _some_ of
the code, that `tc->data[i]`, `tc->data_ext[i]`, and `tc->data_2d[i]`
all contain information about the same "transformable thing". Since
`tc->data` was sorted (by selection state and, optionally for
proportional editing, by distance) but the other arrays were not, this
caused issues.
The most obvious solution, sorting all arrays the same way, turned out
to be hard to do, as some elements in one array have pointers to
elements in another array. Reordering those arrays would therefore
also make it necessary to find and update those pointers.
Instead, I decided to implement a sorted index map. The arrays can
then be kept in their original order, and the index map can be used to
visit them in sorted order.
Pull Request: https://projects.blender.org/blender/blender/pulls/140132
df6d345bb4