Support ScreenCapture loopback #865

Job	Run time
clippy-test	44s
asmjs-wasm32-test (wasm32-unknown-emscripten)	47s
rustfmt-check	8s
cargo-publish	0s
linux-check-and-test-armv7	2m 18s
ubuntu-test	43s
wasm32-bindgen-test (wasm32-unknown-unknown)	28s
wasm32-wasi-test (wasm32-wasi)	15s
windows-test (x86_64)	4m 47s
android-apk-build	1m 31s
android-check	37s
windows-test (i686)	4m 37s
ios-build	1m 59s
macos-test	57s
wasm-beep-build	28s
	20m 19s

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