multiparty/rust: Add Rust version of multiparty demo

Different to the C version this also mixes all participants into a grid
with videomixer.

webrtc/multiparty-sendrecv/gst-rust/Cargo.toml

@@ -0,0 +1,20 @@
+[package]
+name = "webrtc-app"
+version = "0.1.0"
+authors = ["Sebastian Dröge <sebastian@centricular.com>"]
+edition = "2018"
+
+[dependencies]
+futures = "0.3"
+async-std = "1"
+structopt = { version = "0.3", default-features = false }
+anyhow = "1"
+url = "2"
+rand = "0.7"
+async-tungstenite = "0.2"
+gst = { package = "gstreamer", version = "0.14", features = ["v1_14"] }
+gst-webrtc = { package = "gstreamer-webrtc", version = "0.14" }
+gst-sdp = { package = "gstreamer-sdp", version = "0.14", features = ["v1_14"] }
+serde = "1"
+serde_derive = "1"
+serde_json = "1"

webrtc/multiparty-sendrecv/gst-rust/src/macos_workaround.rs

@@ -0,0 +1,67 @@
+/// macOS has a specific requirement that there must be a run loop running
+/// on the main thread in order to open windows and use OpenGL.
+
+#[cfg(target_os = "macos")]
+mod runloop {
+    use std::os::raw::c_void;
+    #[repr(C)]
+    pub struct CFRunLoop(*mut c_void);
+
+    #[link(name = "foundation", kind = "framework")]
+    extern "C" {
+        fn CFRunLoopRun();
+        fn CFRunLoopGetMain() -> *mut c_void;
+        fn CFRunLoopStop(l: *mut c_void);
+    }
+
+    impl CFRunLoop {
+        pub fn run() {
+            unsafe {
+                CFRunLoopRun();
+            }
+        }
+
+        pub fn get_main() -> CFRunLoop {
+            unsafe {
+                let r = CFRunLoopGetMain();
+                assert!(!r.is_null());
+                CFRunLoop(r)
+            }
+        }
+
+        pub fn stop(&self) {
+            unsafe { CFRunLoopStop(self.0) }
+        }
+    }
+
+    unsafe impl Send for CFRunLoop {}
+}
+
+/// On macOS this launches the callback function on a thread.
+/// On other platforms it's just executed immediately.
+#[cfg(not(target_os = "macos"))]
+pub fn run<T, F: FnOnce() -> T + Send + 'static>(main: F) -> T
+where
+    T: Send + 'static,
+{
+    main()
+}
+
+#[cfg(target_os = "macos")]
+pub fn run<T, F: FnOnce() -> T + Send + 'static>(main: F) -> T
+where
+    T: Send + 'static,
+{
+    use std::thread;
+
+    let l = runloop::CFRunLoop::get_main();
+    let t = thread::spawn(move || {
+        let res = main();
+        l.stop();
+        res
+    });
+
+    runloop::CFRunLoop::run();
+
+    t.join().unwrap()
+}