Fix compile errors on Linux (#7527)

Added some missing trait functions and `unimplemented` markings

Release Notes:

- N/A

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

@@ -345,7 +345,9 @@ impl Platform for LinuxPlatform {
     fn set_cursor_style(&self, style: CursorStyle) {}
 
     //todo!(linux)
-    fn should_auto_hide_scrollbars(&self) -> bool {}
+    fn should_auto_hide_scrollbars(&self) -> bool {
+        unimplemented!()
+    }
 
     //todo!(linux)
     fn write_to_clipboard(&self, item: ClipboardItem) {}
@@ -366,6 +368,10 @@ impl Platform for LinuxPlatform {
     fn delete_credentials(&self, url: &str) -> Task<Result<()>> {
         unimplemented!()
     }
+
+    fn window_appearance(&self) -> crate::WindowAppearance {
+        unimplemented!()
+    }
 }
 
 #[cfg(test)]

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);