Switch the BackgroundSizing default to InnerBorderEdge

[robloo]

What does the pull request do?

Switches the default value of BackgroundSizing from CenterBorder to InnerBorderEdge. This aligns Avalonia with the implicit behavior of WPF and the explicit behavior of UWP/WinUI. From a design standpoint it also looks a lot better as the default than CenterBorder.

• https://learn.microsoft.com/en-us/uwp/api/windows.ui.xaml.controls.control.backgroundsizing?view=winrt-26100
• https://learn.microsoft.com/en-us/uwp/api/windows.ui.xaml.controls.border.backgroundsizing?view=winrt-26100

The reason(s) this wasn't done in #14048 are:

1. Backwards compatibility, even with the rendering errors at the time this was the least breaking change for 11.1 compared to 11.0. In 11.0 CenterBorder was the implicit behavior of the simple rendering path.
2. The BoxShadow on Border only works with the simple rendering path, which in turn only works with CenterBorder:
   • Border BoxShadow only works with BackgroundSizing=CenterBorder #15602
   • BoxShadow will not take effect when Border has only one side #11517
   • Border Rendering Issue Roll-up #16387
   • etc.

Now that we are preparing for 12.0 and allowing some breaking changes, it is time to improve the default value and align with other XAML frameworks.

Yes, I expect this to open up a debate.

What is the current behavior?

The current default value is CenterBorder:

What is the updated/expected behavior with this PR?

The default value of BackgroundSizing is now InnerBorderEdge aligning with WPF, UWP and WinUI.

How was the solution implemented (if it's not obvious)?

Obvious.

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

Yes

Obsoletions / Deprecations

None

Fixed issues

None, related to #9946 and #14048