[ShadowContainer] Canvas can be set to a negative Height/Width, crashing on Windows

[kazo0]

Given the content of ShadowContainer that has a Margin whose Top+Bottom or Left+Right is greater than the content's Height or Width can result in a crash on Windows.

With this XAML:

<utu:ShadowContainer>
	<Border Height="50"
			Width="100"
			Background="Red"
			Margin="30" />
</utu:ShadowContainer>

The application will crash with a message of "Value does not fall within the expected range."

The culprit is this line where _canvas.Height will temporarily be given a negative value