Allow RadialGradientBrush to use absolute radius size

[amwx]

Would it be possible to allow the RadialGradientBrush to use an absolute size instead of relative? I'm not sure what's the best solution to offer, but something like RelativePoint or RelativeRect. I'd say RelativeSize, but since Radius is X-Y uniform, that might get confused with the Size struct.

I'm trying to implement a pseudo-Reveal effect, which uses the RadialGradientBrush, and the relative size leads to a non-uniform border effect when things are different sizes, e.g.

[Gillibald]

Looks like this was already documented but somehow got lost:https://github.com/AvaloniaUI/Avalonia/blob/master/src/Avalonia.Visuals/Media/RadialGradientBrush.cs#L57

[grokys]

Speaking about this with @wieslawsoltes and looks like this property controls this on WPF: https://docs.microsoft.com/en-us/dotnet/api/system.windows.media.gradientbrush.mappingmode?view=net-5.0