[ShadowContainer] support complex shapes

[kucint]

Current behavior

When adding a shadow to the line, a boundary rectangle is used to create a shadow

<Page x:Class="UnoShadowContainerApp.MainPage"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      xmlns:local="using:UnoShadowContainerApp"
      xmlns:utu="using:Uno.Toolkit.UI"
      Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
  <ScrollViewer>
    <Grid HorizontalAlignment="Center" VerticalAlignment="Center">
      <Grid.ColumnDefinitions>
        <ColumnDefinition Width="200" />
        <ColumnDefinition Width="200" />
      </Grid.ColumnDefinitions>

      <utu:ShadowContainer Grid.Column="0" HorizontalAlignment="Center">
        <utu:ShadowContainer.Shadows>
          <utu:Shadow
            BlurRadius="30"
            Spread="-1"
            Opacity="0.5"
            Color="lightGreen" />
        </utu:ShadowContainer.Shadows>
        <StackPanel
          HorizontalAlignment="Center"
          VerticalAlignment="Center">
          <Rectangle
            Width="100"
            Height="80"
            Stroke="Black"
            StrokeThickness="1"
            RadiusX="4"
            RadiusY="4"
            Fill="LightBlue" />
          <TextBlock
            Text="Hello Uno Platform!"
            HorizontalAlignment="Center" />
        </StackPanel>
      </utu:ShadowContainer>

      <utu:ShadowContainer Grid.Column="1" HorizontalAlignment="Center">
        <utu:ShadowContainer.Shadows>
          <utu:Shadow
            BlurRadius="50"
            Spread="-1"
            Opacity="0.5"
            Color="lightGreen" />
        </utu:ShadowContainer.Shadows>
          <Line
            X1="0"
            Y1="0"
            X2="100"
            Y2="100"
            Stroke="Black"
            StrokeThickness="1" />
      </utu:ShadowContainer>
    </Grid>
  </ScrollViewer>
</Page>

Expected behavior

I expect that line will be "shadowed" rather than its boundary rectangle.

Important

in my project I want to add a shadow to an object that is much more complex than a simple line. It could look like as follow:

The shadow shall be around the presented Path rather than around Path's boundary rectangle.

I yes, I know, this is not a trivial problem....
Any idea how to work around it?

How to reproduce it (as minimally and precisely as possible)

MINIMAL REPRO PROJECT: UnoShadowContainerApp.zip

Nuget Package:

"Uno.Sdk": "5.5.54"
<PackageVersion Include="Uno.Toolkit.Skia.WinUI" Version="6.3.6" />

Package Version(s):

Affected platform(s):

• WebAssembly
• Android
• iOS
• macOS (AppKit)
• Mac Catalyst
• Skia
  • WPF
  • GTK (Linux)
  • Linux Framebuffer
  • Tizen
• Windows

IDE:

• Visual Studio 2022
• Visual Studio 2019
• Visual Studio Code
• Visual Studio for Mac
• Rider Windows
• Rider macOS

Relevant plugins:

Anything else we need to know?

[Xiaoy312]

Unfortunately, this is not supported at the moment.

Remarks

Currently, the control simply mimics the shape of its content by size and by corner radius. Complicated shapes, such as text or pictures with alpha, are not yet supported.
-- https://github.com/unoplatform/uno.toolkit.ui/blob/main/doc/controls/ShadowContainer.md#remarks