There were lot of places, where timeline frame was incorrectly
mapped to frame index.
For deleting transitions, internal function `seq_retiming_add_key`
was added. It can work in frame index domain directly. This is to avoid
adding more complexity, since key positions are stored and re-created
later. Going through index to frame and back to index would introduce
float precision errors.
Pull Request: https://projects.blender.org/blender/blender/pulls/129305
9ef6136865