Support ScreenCapture loopback #940

Job	Run time
ubuntu-test	31s
clippy-test	47s
linux-check-and-test-armv7	1m 54s
asmjs-wasm32-test (wasm32-unknown-emscripten)	1m 3s
cargo-publish	0s
macos-test	1m 46s
rustfmt-check	11s
wasm32-bindgen-test (wasm32-unknown-unknown)	29s
wasm32-wasi-test (wasm32-wasi)	22s
windows-test (x86_64)	4m 28s
windows-test (i686)	2m 14s
android-apk-build	1m 37s
android-check	39s
ios-build	2m 20s
wasm-beep-build	34s
	18m 55s

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