Support merging UV's that share the same vertex and are very close when
applying modifiers.
This is needed to prevent UV's becoming "detached" which can happen when
applying the subdivision surface modifier.
This regression was caused by [0] which removed selection threshold for
nearby coordinates. While restoring the UV selection threshold could be
done - some selection operations that walk around connected UV fans
wouldn't behave in a deterministic way (such as select shortest path).
There are also other cases where UV's may be compared without a
threshold such as tangent calculation and exporters which have their own
logic to handling UV's.
Also resolves T86896, T89903.
[0]: b88dd3b8e7
Reviewed By: sergey
Ref D14841
78fc5ea1c3