Support piped input with QtAVPlayer #61

Job	Run time
build-macos-no-brew (clang-9)	4m 40s
build-macos-brew-Qt6 (clang-9)	3m 18s
build-macos-brew (clang-9)	2m 25s
build-linux-Qt6 (gcc-9)	7m 36s
build-macos-no-brew-Qt6 (clang-9)	5m 47s
build-windows-mingw (5.15.*)	9m 45s
build-windows-mingw-Qt6 (6.3.0, qtmultimedia qt5compat)	1m 43s
build-windows-vs (5.15.*)	8m 51s
build-windows-vs-Qt6 (6.3.0, qtmultimedia qt5compat)	12m 37s
build-linux (gcc-9)	7m 11s
	1h 3m 53s

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