Use scap library to implement screensharing on X11 (#27807)

While `scap` does have support for Wayland and Windows, but haven't seen
screensharing work properly there yet. So for now just adding support
for X11 screensharing.

WIP branches for enabling wayland and windows support:

* https://github.com/zed-industries/zed/tree/wayland-screenshare
* https://github.com/zed-industries/zed/tree/windows-screenshare


Release Notes:

- Added support for screensharing on X11 (Linux)

---------

Co-authored-by: Conrad <conrad@zed.dev>
Co-authored-by: Mikayla <mikayla@zed.dev>
Co-authored-by: Junkui Zhang <364772080@qq.com>

crates/gpui/src/platform/linux/headless/client.rs

@@ -1,13 +1,16 @@
 use std::cell::RefCell;
 use std::rc::Rc;
 
+use anyhow::anyhow;
 use calloop::{EventLoop, LoopHandle};
-
+use futures::channel::oneshot;
 use util::ResultExt;
 
 use crate::platform::linux::LinuxClient;
 use crate::platform::{LinuxCommon, PlatformWindow};
-use crate::{AnyWindowHandle, CursorStyle, DisplayId, PlatformDisplay, WindowParams};
+use crate::{
+    AnyWindowHandle, CursorStyle, DisplayId, PlatformDisplay, ScreenCaptureSource, WindowParams,
+};
 
 pub struct HeadlessClientState {
     pub(crate) _loop_handle: LoopHandle<'static, HeadlessClient>,
@@ -63,6 +66,21 @@ impl LinuxClient for HeadlessClient {
         None
     }
 
+    fn is_screen_capture_supported(&self) -> bool {
+        false
+    }
+
+    fn screen_capture_sources(
+        &self,
+    ) -> oneshot::Receiver<anyhow::Result<Vec<Box<dyn ScreenCaptureSource>>>> {
+        let (mut tx, rx) = oneshot::channel();
+        tx.send(Err(anyhow!(
+            "Headless mode does not support screen capture."
+        )))
+        .ok();
+        rx
+    }
+
     fn active_window(&self) -> Option<AnyWindowHandle> {
         None
     }