Fix: don't interact with `Area` outside its `constrain_rect` (#5459)

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This PR makes an area's interact rect intersect its constrain rect. This
fixes an issue where masked areas would still intercept input.


Before:

![before](https://github.com/user-attachments/assets/6b790a04-8a15-44fe-a7ae-4adda189ecba)

After:

![after](https://github.com/user-attachments/assets/98010d89-e680-44cb-8717-faf31b0912d3)

crates/egui/src/containers/area.rs

@@ -467,7 +467,7 @@ impl Area {
                     id: interact_id,
                     layer_id,
                     rect: state.rect(),
-                    interact_rect: state.rect(),
+                    interact_rect: state.rect().intersect(constrain_rect),
                     sense,
                     enabled,
                 },