gpui: Make screen capture dependency optional (#32937)

Add `screen-capture` feature to gpui to enable screen capture support.  The motivation for this is to make dependencies on scap / x11 / xcb optional.

Release Notes:

- N/A

---------

Co-authored-by: Michael Sloan <michael@zed.dev>

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

@@ -1,16 +1,14 @@
 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, LinuxKeyboardLayout, PlatformDisplay,
-    PlatformKeyboardLayout, ScreenCaptureSource, WindowParams,
+    PlatformKeyboardLayout, WindowParams,
 };
 
 pub struct HeadlessClientState {
@@ -67,15 +65,18 @@ impl LinuxClient for HeadlessClient {
         None
     }
 
+    #[cfg(feature = "screen-capture")]
     fn is_screen_capture_supported(&self) -> bool {
         false
     }
 
+    #[cfg(feature = "screen-capture")]
     fn screen_capture_sources(
         &self,
-    ) -> oneshot::Receiver<anyhow::Result<Vec<Box<dyn ScreenCaptureSource>>>> {
-        let (mut tx, rx) = oneshot::channel();
-        tx.send(Err(anyhow!(
+    ) -> futures::channel::oneshot::Receiver<anyhow::Result<Vec<Box<dyn crate::ScreenCaptureSource>>>>
+    {
+        let (mut tx, rx) = futures::channel::oneshot::channel();
+        tx.send(Err(anyhow::anyhow!(
             "Headless mode does not support screen capture."
         )))
         .ok();