Border Rendering Issue Roll-up

[robloo]

Describe the bug

This is a roll-up of all Border-related issues that should be addressed next.

1. Border does not clip it's children. This becomes an issue for rounded corners especially after adding BackgroundSizing and we should follow UWP/WinUI here.
   • Support Clipping to rounded corners #2105
   • Complexity is not that high to close this since we now have all the clipping geometry already calculated (for BackgroundSizing). The concern is more is this a breaking change and if so when can we adopt it.
2. Border.BoxShadow only ever worked with the simple rendering path (we let Skia do everything). With the advanced rendering path it doesn't work. This has been the case for quite some time but again is more noticeable with BackgroundSizing. Anything other than CenterBorder will force the complex rendering path and therefore loose the BoxShadow
   • Border BoxShadow only works with BackgroundSizing=CenterBorder #15602
   • This one is probably the most difficult to solve. It likely involves moving away from Skia doing this and switching to more generalized effects: Border BoxShadow only works with BackgroundSizing=CenterBorder #15602 (comment)
3. Corner radii greater than the height/width of a Border render differently depending on the simple/complex rendering path. Again, in the simple path we fallback to Skia which has a different behavior than the advanced rendering path (which works like WPF/UPW/WinUI).
   • BackgroundSizing Inner/Outer do not work like Center when CornerRadius is very large (i.e. 999) #15875
   • We need to stop letting Skia render corners in this case. This likely means we just need to remove the simple rendering path entirely (probably needed for BoxShadow issues above anyway).
   • We should be following what is done in WPF/UPW/WinUI here (and likely DirectX as well).

To Reproduce

See each issue specifically

Expected behavior

No response

Avalonia version

11.1+

OS

Windows

Additional context

I intend to close these myself if others don't beat me to it. The timeframe for some may be 12.0.