Implement Feature: Stretch Subtitles Timing by Factor

[Matt McManis]

I'm looking to have a feature added, Stretch Subtitles by a factor [0.5, 100.0] range, like how atempo or rubberband works.

https://ffmpeg.org/ffmpeg-filters.html#atempo
https://ffmpeg.org/ffmpeg-filters.html#toc-rubberband

Example: Something like -substretch 3.0 (speed up 300%), -substretch 0.5 (slow down 50%).
Or as a filter -vf "subtitles=stretch=3.0".

The program MKVToolNix is able to do this under the "Timestamps and default duration" section.

This would be different that itsoffset, which does not stretch, but shifts.