Mask definitions are not supported

[steveharoz]

I'm not sure if it's feasible to fix. But it's worth stating that this feature won't work.

[jogibear9988]

as it is possible in xaml:
https://docs.microsoft.com/de-de/dotnet/framework/wpf/graphics-multimedia/opacity-masks-overview
we should be able to support it in the control

[jogibear9988]

fixed