9cb5285c0a
This un-breaks libqtav by upgrading it to the current upstream HEAD.
Previously, Qtav was broken by the Qt 5.12 -> 5.14 migration (#84232).
See commit 819bb63d5c
This also un-pins ffmpeg_3; libqtav works fine with current ffmpeg.
Digikam is the only derivation in nixpkgs that has referenced libqtav, thus
upgrading to a pre-release revision poses minimal risk.