Allow platform references to be sent to background threads

Co-Authored-By: Max Brunsfeld <maxbrunsfeld@gmail.com>

gpui/src/platform/mac.rs

@@ -12,15 +12,15 @@ use cocoa::base::{BOOL, NO, YES};
 pub use dispatcher::Dispatcher;
 pub use fonts::FontSystem;
 use platform::{MacMainThreadPlatform, MacPlatform};
-use std::rc::Rc;
+use std::{rc::Rc, sync::Arc};
 use window::Window;
 
 pub(crate) fn main_thread_platform() -> Rc<dyn super::MainThreadPlatform> {
     Rc::new(MacMainThreadPlatform::default())
 }
 
-pub(crate) fn platform() -> Rc<dyn super::Platform> {
-    Rc::new(MacPlatform::new())
+pub(crate) fn platform() -> Arc<dyn super::Platform> {
+    Arc::new(MacPlatform::new())
 }
 
 trait BoolExt {