Opt-out for UI clipping (#19826)

# Objective

Opt-out for UI clipping, for motivation see issue #19821

## Solution

New zst component `OverrideClip`. A UI node entity with this component
will ignore any inherited clipping rect, so it will never get clipped
regardless of the `Overflow` settings of its ancestors.

#### Why use a marker component and not add a new variant to `Overflow`
instead?
A separate marker component allows users to set both `Overflow` and
`OverrideClip` on the same node.

## Testing

Run the `overflow` example with the `OverrideClip` component added to
the `ImagNode`s and you will see that clipping is disabled.

crates/bevy_ui/src/ui_node.rs

@@ -2246,6 +2246,11 @@ pub struct CalculatedClip {
     pub clip: Rect,
 }
 
+/// UI node entities with this component will ignore any clipping rect they inherit,
+/// the node will not be clipped regardless of its ancestors' `Overflow` setting.
+#[derive(Component)]
+pub struct OverrideClip;
+
 /// Indicates that this [`Node`] entity's front-to-back ordering is not controlled solely
 /// by its location in the UI hierarchy. A node with a higher z-index will appear on top
 /// of sibling nodes with a lower z-index.

crates/bevy_ui/src/update.rs

@@ -3,8 +3,8 @@
 use crate::{
     experimental::{UiChildren, UiRootNodes},
     ui_transform::UiGlobalTransform,
-    CalculatedClip, ComputedNodeTarget, DefaultUiCamera, Display, Node, OverflowAxis, UiScale,
+    CalculatedClip, ComputedNodeTarget, DefaultUiCamera, Display, Node, OverflowAxis, OverrideClip,
-    UiTargetCamera,
+    UiScale, UiTargetCamera,
 };
 
 use super::ComputedNode;
@@ -12,7 +12,7 @@ use bevy_ecs::{
     change_detection::DetectChangesMut,
     entity::Entity,
     hierarchy::ChildOf,
-    query::{Changed, With},
+    query::{Changed, Has, With},
     system::{Commands, Query, Res},
 };
 use bevy_math::{Rect, UVec2};
@@ -28,6 +28,7 @@ pub fn update_clipping_system(
         &ComputedNode,
         &UiGlobalTransform,
         Option<&mut CalculatedClip>,
+        Has<OverrideClip>,
     )>,
     ui_children: UiChildren,
 ) {
@@ -50,15 +51,22 @@ fn update_clipping(
         &ComputedNode,
         &UiGlobalTransform,
         Option<&mut CalculatedClip>,
+        Has<OverrideClip>,
     )>,
     entity: Entity,
     mut maybe_inherited_clip: Option<Rect>,
 ) {
-    let Ok((node, computed_node, transform, maybe_calculated_clip)) = node_query.get_mut(entity)
+    let Ok((node, computed_node, transform, maybe_calculated_clip, has_override_clip)) =
+        node_query.get_mut(entity)
     else {
         return;
     };
 
+    // If the UI node entity has an `OverrideClip` component, discard any inherited clip rect
+    if has_override_clip {
+        maybe_inherited_clip = None;
+    }
+
     // If `display` is None, clip the entire node and all its descendants by replacing the inherited clip with a default rect (which is empty)
     if node.display == Display::None {
         maybe_inherited_clip = Some(Rect::default());