Fix compile errors on Linux (#7527)

Added some missing trait functions and `unimplemented` markings

Release Notes:

- N/A

crates/gpui/src/platform/linux/window.rs

@@ -410,9 +410,6 @@ impl PlatformWindow for LinuxWindow {
         unimplemented!()
     }
 
-    //todo!(linux)
-    fn invalidate(&self) {}
-
     fn draw(&self, scene: &crate::Scene) {
         let mut inner = self.0.inner.lock();
         inner.renderer.draw(scene);