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/media/Cargo.toml

@@ -17,6 +17,7 @@ anyhow.workspace = true
 
 [target.'cfg(target_os = "macos")'.dependencies]
 core-foundation.workspace = true
+ctor.workspace = true
 foreign-types = "0.5"
 metal = "0.29"
 objc = "0.2"

crates/media/src/media.rs

@@ -253,11 +253,14 @@ pub mod core_media {
             }
         }
 
-        pub fn image_buffer(&self) -> CVImageBuffer {
+        pub fn image_buffer(&self) -> Option<CVImageBuffer> {
             unsafe {
-                CVImageBuffer::wrap_under_get_rule(CMSampleBufferGetImageBuffer(
-                    self.as_concrete_TypeRef(),
-                ))
+                let ptr = CMSampleBufferGetImageBuffer(self.as_concrete_TypeRef());
+                if ptr.is_null() {
+                    None
+                } else {
+                    Some(CVImageBuffer::wrap_under_get_rule(ptr))
+                }
             }
         }