MacOS: cache custom cursors (#3291)

src/platform_impl/macos/cursor.rs

@@ -0,0 +1,17 @@
+use objc2::rc::Id;
+
+use super::appkit::NSCursor;
+use super::EventLoopWindowTarget;
+use crate::cursor::OnlyCursorImageBuilder;
+
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
+pub struct CustomCursor(pub(crate) Id<NSCursor>);
+
+impl CustomCursor {
+    pub(crate) fn build<T>(
+        cursor: OnlyCursorImageBuilder,
+        _: &EventLoopWindowTarget<T>,
+    ) -> CustomCursor {
+        Self(NSCursor::from_image(&cursor.0))
+    }
+}