Simplify logic of speed effect frame calculation by using discrete math
where possible. Only `SEQ_SPEED_MULTIPLY` mode with animation requires
frame map to be built. Frame map building was simplified by removing
unused branches.
Functional change: Animating strip in negative range will reverse playback.
I assume this was limitation of previous system, where each frame map item
was limited to be within correct frame range. Now frame map can contain
values that point beyond usable range and they are limited by
`seq_speed_effect_target_frame_get`. This way it is possible to control
playback rate in both directions.
Mostly fixes T89120 apart from offset handling.
Reviewed By: mano-wii
Differential Revision: https://developer.blender.org/D11939
**Drawing Changes:**
- F-curve drawing for Stretch, Multiply, Length and Frame Number.
- Value drawing when no keyframes for Stretch, Length and Frame Numbers.
General view of the new drawing for each speed effect mode:
{F9796642, size=full}
Detail of the horizontal zero (blue) line in the new `Multiply` mode:
{F9798520, size=full}
Nice to have (but I don't know how):
- Auto adjusting of endframe when using Multiply or Boost.
Differential Revision: https://developer.blender.org/D6110
Move most functions into separate files.
File sequencer.c is reserved for alloc, free and duplicating functions.
There should be no functional changes.
