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/mac.rs

@@ -4,12 +4,14 @@ mod dispatcher;
 mod display;
 mod display_link;
 mod events;
+mod screen_capture;
 
 #[cfg(not(feature = "macos-blade"))]
 mod metal_atlas;
 #[cfg(not(feature = "macos-blade"))]
 pub mod metal_renderer;
 
+use media::core_video::CVImageBuffer;
 #[cfg(not(feature = "macos-blade"))]
 use metal_renderer as renderer;
 
@@ -49,6 +51,9 @@ pub(crate) use window::*;
 #[cfg(feature = "font-kit")]
 pub(crate) use text_system::*;
 
+/// A frame of video captured from a screen.
+pub(crate) type PlatformScreenCaptureFrame = CVImageBuffer;
+
 trait BoolExt {
     fn to_objc(self) -> BOOL;
 }