fix occasional crash moving ui root nodes (#11371)

# Objective

fix an occasional crash when moving ui root nodes between cameras.

occasionally, updating the TargetCamera of a ui element and then
removing the element causes a crash.

i believe that is because when we assign a child in taffy, the old
parent doesn't remove that child from it's children, so we have:

```
user: create root node N1, camera A
-> layout::set_camera_children(A) : 
	- create implicit node A1
	- assign 1 as child -> taffy.children[A1] = [N1], taffy.parents[1] = A1

user: move root node N1 to camera B
-> layout::set_camera_children(B) :
	- create implicit node B1
	- assign 1 as child -> taffy.children[A1] = [N1], taffy.children[B1] = [N1], taffy.parents[1] = B1
-> layout::set_camera_children(A) :
	- remove implicit node A1 (which still has N1 as a child) -> 
		-> taffy sets parent[N1] = None ***
		-> taffy.children[B1] = [N1], taffy.parents[1] = None

user: remove N1
-> layout::remove_entities(N1)
	- since parent[N1] is None, it's not removed from B1 -> taffy.children[B1] = [N1], taffy.parents[1] is removed
-> layout::set_camera_children(B)
	- remove implicit node B1
	- taffy crash accessing taffy.parents[N1]
```

## Solution

we can work around this by making sure to remove the child from the old
parent if one exists (this pr).

i think a better fix may be for taffy to check in `Taffy::remove` and
only set the child's parent to None if it is currently equal to the node
being removed but i'm not sure if there's an explicit assumption we're
violating here (@nicoburns).

Author: robtfm

crates/bevy_ui/src/layout/mod.rs

@@ -19,7 +19,7 @@ use bevy_transform::components::Transform;
 use bevy_utils::{default, EntityHashMap, HashMap, HashSet};
 use bevy_window::{PrimaryWindow, Window, WindowScaleFactorChanged};
 use std::fmt;
-use taffy::Taffy;
+use taffy::{tree::LayoutTree, Taffy};
 use thiserror::Error;
 
 pub struct LayoutContext {
@@ -169,12 +169,19 @@ without UI components as a child of an entity with UI components, results may be
                 .iter()
                 .find(|n| n.user_root_node == node)
                 .cloned()
-                .unwrap_or_else(|| RootNodePair {
-                    implicit_viewport_node: self
-                        .taffy
-                        .new_with_children(viewport_style.clone(), &[node])
-                        .unwrap(),
-                    user_root_node: node,
+                .unwrap_or_else(|| {
+                    if let Some(previous_parent) = self.taffy.parent(node) {
+                        // remove the root node from the previous implicit node's children
+                        self.taffy.remove_child(previous_parent, node).unwrap();
+                    }
+
+                    RootNodePair {
+                        implicit_viewport_node: self
+                            .taffy
+                            .new_with_children(viewport_style.clone(), &[node])
+                            .unwrap(),
+                        user_root_node: node,
+                    }
                 });
             new_roots.push(root_node);
         }