This feature implements smooth transition between 2 retimed segments.
A transition can be added by dragging retiming handle while holding
shift key. When any handle defining transition is removed, the
transition segment is removed and 2 linear segments are restored to
state before transition was created.
These transitions work with video and sound and changes are reflected
in waveforms with correct seeking, therefore it replaces usage of
`speed_factor` or earlier sound `pitch` property.
Smooth transition is achieved by evaluating Y value of circular arc,
that is tangent to lines of linear retimed segments. Because of this,
when transition length is changed both handles are moved symetrically.
This way it is possible to make transition without affecting content in
linear segments.
When linear segment after transition is further retimed, this shifts
content inside of a transition segment.
To support sound, `SEQ_retiming_sound_animation_data_set` and related
data structures has been updated to build speed table for each smooth
transition segment. Layering of retiming data via meta strips is still
fully supported.
Pull Request: https://projects.blender.org/blender/blender/pulls/107197
4dc026ec8e