Use LiveKit's Rust SDK on Linux while continue using Swift SDK on Mac (#21550)

Similar to #20826 but keeps the Swift implementation. There were quite a
few changes in the `call` crate, and so that code now has two variants.

Closes #13714

Release Notes:

- Added preliminary Linux support for voice chat and viewing
screenshares.

---------

Co-authored-by: Kirill Bulatov <mail4score@gmail.com>
Co-authored-by: Kirill Bulatov <kirill@zed.dev>
Co-authored-by: Mikayla <mikayla@zed.dev>

crates/gpui/src/platform.rs

@@ -71,6 +71,9 @@ pub(crate) use test::*;
 #[cfg(target_os = "windows")]
 pub(crate) use windows::*;
 
+#[cfg(any(test, feature = "test-support"))]
+pub use test::TestScreenCaptureSource;
+
 #[cfg(target_os = "macos")]
 pub(crate) fn current_platform(headless: bool) -> Rc<dyn Platform> {
     Rc::new(MacPlatform::new(headless))
@@ -150,6 +153,10 @@ pub(crate) trait Platform: 'static {
         None
     }
 
+    fn screen_capture_sources(
+        &self,
+    ) -> oneshot::Receiver<Result<Vec<Box<dyn ScreenCaptureSource>>>>;
+
     fn open_window(
         &self,
         handle: AnyWindowHandle,
@@ -229,6 +236,25 @@ pub trait PlatformDisplay: Send + Sync + Debug {
     }
 }
 
+/// A source of on-screen video content that can be captured.
+pub trait ScreenCaptureSource {
+    /// Returns the video resolution of this source.
+    fn resolution(&self) -> Result<Size<Pixels>>;
+
+    /// Start capture video from this source, invoking the given callback
+    /// with each frame.
+    fn stream(
+        &self,
+        frame_callback: Box<dyn Fn(ScreenCaptureFrame)>,
+    ) -> oneshot::Receiver<Result<Box<dyn ScreenCaptureStream>>>;
+}
+
+/// A video stream captured from a screen.
+pub trait ScreenCaptureStream {}
+
+/// A frame of video captured from a screen.
+pub struct ScreenCaptureFrame(pub PlatformScreenCaptureFrame);
+
 /// An opaque identifier for a hardware display
 #[derive(PartialEq, Eq, Hash, Copy, Clone)]
 pub struct DisplayId(pub(crate) u32);