Submerge 3.9

- Now using AVFoundation to export MPEG-4 files and FFmpeg for everything else.
- AVFoundation uses hardware encoding and it's really, really fast.
- FFmpeg is versatile and can export different formats like .MKV and .AVI
- There's a setting in Preferences to always use FFmpeg for export if wanted.
- FFmpeg updated to version 5.0 "Lorentz".