Change the default visual box for OverflowClipMargin to PaddingBox (#18935)

# Objective

The default should be `OverflowClipBox::PaddingBox` not
`OverflowClipBox::ContentBox`

`padding-box` is the default in CSS. 

## Solution

Set the default to `PaddingBox`.

## Testing

Compare the `overflow` UI example on main vs with this PR. You should
see that on main the outline around the inner node gets clipped. With
this PR by default clipping starts at the inner edge of the border (the
`padding-box`) and the outlines are visible.

Fixes #18934

Author: ickshonpe

crates/bevy_ui/src/ui_node.rs

@@ -1178,7 +1178,7 @@ pub struct OverflowClipMargin {
 
 impl OverflowClipMargin {
     pub const DEFAULT: Self = Self {
-        visual_box: OverflowClipBox::ContentBox,
+        visual_box: OverflowClipBox::PaddingBox,
         margin: 0.,
     };
 
@@ -1224,9 +1224,9 @@ impl OverflowClipMargin {
 )]
 pub enum OverflowClipBox {
     /// Clip any content that overflows outside the content box
-    #[default]
     ContentBox,
     /// Clip any content that overflows outside the padding box
+    #[default]
     PaddingBox,
     /// Clip any content that overflows outside the border box
     BorderBox,

release-content/migration-guides/overflowclipbox_default_is_now_paddingbox.md

@@ -0,0 +1,7 @@
+---
+title: OverflowClipBox's default is now Paddingbox
+pull_requests: [18935]
+---
+
+The default variant for `OverflowClipBox` is now `PaddingBox`.
+The default value for `OverflowClipMargin::visual_box` is now `OverflowClipBox::PaddingBox`.