MacOS: cache custom cursors (#3291)

src/platform_impl/macos/window.rs

@@ -849,7 +849,7 @@ impl WinitWindow {
     #[inline]
     pub(crate) fn set_custom_cursor(&self, cursor: PlatformCustomCursor) {
         let view = self.view();
-        view.set_cursor_icon(NSCursor::from_image(&cursor.0));
+        view.set_cursor_icon(cursor.0.clone());
         self.invalidateCursorRectsForView(&view);
     }