Matroska capture misc fixes #224

Job	Run time
build-macos-brew (clang-9)	7m 30s
build-linux-Qt6 (gcc-9)	6m 20s
build-linux (gcc-9)	5m 21s
build-macos-brew-Qt6 (clang-9)	9m 19s
build-macos-no-brew (clang-9)	16m 58s
build-macos-no-brew-Qt6 (clang-9)	17m 0s
build-windows-mingw (5.15.2)	15m 51s
build-windows-mingw-Qt6 (6.3.0, qtmultimedia qt5compat)	18m 11s
build-windows-vs (5.15.2)	12m 35s
build-windows-vs-Qt6 (6.3.0, qtmultimedia qt5compat)	19m 26s
	2h 8m 31s

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