Add build of LibFFmpeg as submodule #971

Job	Run time
build-mac-native (macos-12)	1m 42s
build-unix (ubuntu-20.04, qtBase_6.5.2_ubuntu20.zip, libde265.so, libde265-internals.so, YUView.U...	29s
build-unix-native (ubuntu-20.04, libgl1-mesa-dev libxkbcommon-x11-0 libpcre2-16-0 qt5-default, qm...	28s
build-unix (macos-11, qtBase_6.5.2_mac11.zip, libde265.dylib, libde265-internals.dylib, YUView-Ma...	41s
build-windows (windows-2019, true, YUView-Win2019.zip, qtBase_6.5.2_win2019.zip)	57s
build-unix-native (ubuntu-22.04, qt6-base-dev, qmake6)	29s
build-unix (macos-12, qtBase_6.5.2_mac12.zip, libde265.dylib, libde265-internals.dylib, YUView-Ma...	44s
build-windows (windows-2019, false, YUView-Win2019-noautoupdate.zip, qtBase_6.5.2_win2019.zip)	57s
	6m 27s

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