Property `speed_factor` was used before retiming. It was kept for
potential versioning code for complex speed animation, that was
possible with even older `pitch` property.
Strips, where `speed_factor` is set to static value are already
correctly converted.
Such versioning code would be quite complex, possibly slow and maybe
could corrupt files. This is due to multiple factors:
- Sound seeking was broken, so conversion would have to ignore all
keyframes before strip starts to map frames properly.
- Because some animation was effectively ignored, it may cause
inconsistencies when doing conversion.
- It would have to integrate value of `speed_factor` to map keyframes
to strip space, but the animation is not limited to strip length.
- For each keyframe where speed gradually changes, at least one smooth
speed transition would be required, but there would be discrepancies
that would have to be accounted for. With simpler strategy it is
likely, that extent of ramps would be limited and thus animation would
be quite different from original.
Because of these reasons, I think it is best to not convert
`speed_factor` animation.
Pull Request: https://projects.blender.org/blender/blender/pulls/114295
01b617a3c6