Fixes#142735.
The original clamp patch in #134319 did not add any cases for retiming
key transformation code. A simple solution to the crash would be to
disable the behavior, but this PR opts to bring feature parity to
retiming keys too.
This means that header values are now more accurate, and snap lines do
not erroneously draw when retiming keys get clamped.
It also changes transition key transformation slightly, aligning the
direction of movement with the movement of the two keys. Before, moving
the mouse backwards would have the same effect, which was clunky and
unintuitive.
Pull Request: https://projects.blender.org/blender/blender/pulls/142809
6571236b59