Matroska output fixes #217

Job	Run time
build-linux (gcc-9)	5m 34s
build-linux-Qt6 (gcc-9)	6m 39s
build-macos-brew (clang-9)	10m 31s
build-macos-no-brew (clang-9)	21m 9s
build-macos-brew-Qt6 (clang-9)	6m 25s
build-macos-no-brew-Qt6 (clang-9)	15m 38s
build-windows-mingw (5.15.2)	15m 53s
build-windows-mingw-Qt6 (6.3.0, qtmultimedia qt5compat)	18m 1s
build-windows-vs (5.15.2)	12m 38s
build-windows-vs-Qt6 (6.3.0, qtmultimedia qt5compat)	18m 50s
	2h 11m 18s

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