This uses Fluent UI System Icons [1] as a source for icons. The glyps
are generated using `TOOLS/gen-fluent-glyphs.py` script, which downloads
Icon and convert for our mpv font to use. Glyphs are vendored in our
repository to avoid external dependency.
Script could be extended to add more icons/variants easily, currently
only filled version are imported.
[1] https://github.com/microsoft/fluentui-system-icons