Remove debug-assert triggered by with_layer_id/dnd_drag_source (#5191)

* Closes #5178

crates/egui/src/ui.rs

@@ -2260,6 +2260,16 @@ impl Ui {
     }
 
     /// Redirect shapes to another paint layer.
+    ///
+    /// ```
+    /// # use egui::{LayerId, Order, Id};
+    /// # egui::__run_test_ui(|ui| {
+    /// let layer_id = LayerId::new(Order::Tooltip, Id::new("my_floating_ui"));
+    /// ui.with_layer_id(layer_id, |ui| {
+    ///     ui.label("This is now in a different layer");
+    /// });
+    /// # });
+    /// ```
     pub fn with_layer_id<R>(
         &mut self,
         layer_id: LayerId,

crates/egui/src/widget_rect.rs

@@ -139,11 +139,6 @@ impl WidgetRects {
                 // e.g. calling `response.interact(…)` to add more interaction.
                 let (idx_in_layer, existing) = entry.get_mut();
 
-                debug_assert!(
-                    existing.layer_id == widget_rect.layer_id,
-                    "Widget changed layer_id during the frame"
-                );
-
                 // Update it:
                 existing.rect = widget_rect.rect; // last wins
                 existing.interact_rect = widget_rect.interact_rect; // last wins