Add new options for -speed-mode

[Ahed91]

This project is very interesting 👍
so i start testing with some videos but i have to wait a long time so if possible to add at least more options for -speed-mode like (Presets in H.264)
https://trac.ffmpeg.org/wiki/Encode/H.264#Preset
faster - fast - medium

[jonatansamuelsson]

Thank you for highlighting that faster speed modes would be desirable to be able to produce xvc bitstreams quicker. We are continuously working on improving the performance of the encoder and faster speed modes is a feature with relatively high priority but we cannot make any promises around when it will be available.