Matroska capture misc fixes #226

Job	Run time
build-macos-brew (clang-9)	6m 19s
build-macos-brew-Qt6 (clang-9)	6m 21s
build-linux (gcc-9)	5m 12s
build-linux-Qt6 (gcc-9)	6m 31s
build-macos-no-brew (clang-9)	11m 46s
build-macos-no-brew-Qt6 (clang-9)	15m 10s
build-windows-mingw (5.15.2)	15m 18s
build-windows-mingw-Qt6 (6.3.0, qtmultimedia qt5compat)	18m 13s
build-windows-vs (5.15.2)	12m 25s
build-windows-vs-Qt6 (6.3.0, qtmultimedia qt5compat)	19m 2s
	1h 56m 17s

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