Build 21

Linux x86_64 release of Marker (egui + MuPDF).

Commit: 964477145581fdfe07610ade0e763e7bab801776

Runtime shared libraries typically needed on the host:
fontconfig, freetype, brotli, jbig2dec, lcms2, libjpeg, openjpeg2, zlib.

License: MIT.

What's Changed

• fix(#29): Single-instance open as tabs + move to new window by @danielschafi in #50
• fix(#30): Keep highlight text readable by @danielschafi in #43
• fix(#25): Rework right-click context menu by @danielschafi in #45

Full Changelog: build-18...build-21