Support piped input with QtAVPlayer #60

Job	Run time
build-macos-no-brew-Qt6 (clang-9)	5m 42s
build-linux (gcc-9)	6m 12s
build-macos-brew (clang-9)	1m 55s
build-macos-brew-Qt6 (clang-9)	2m 45s
build-linux-Qt6 (gcc-9)	7m 30s
build-windows-mingw (5.15.*)	5m 20s
build-windows-mingw-Qt6 (6.3.0, qtmultimedia qt5compat)	11m 28s
build-windows-vs (5.15.*)	5m 43s
build-windows-vs-Qt6 (6.3.0, qtmultimedia qt5compat)	12m 47s
build-macos-no-brew (clang-9)	4m 56s
	1h 4m 18s

Provide feedback