DropShadowPanel should listen to SizeChanged events on its casting element

[seanmcr]

Today it's possible for the DropShadowPanel's Content/casting element to change sizes without triggering an update to the shadow size...for instance, consider a DropShadowPanel whose HorizontalContentAlignment is set to Left, and then sometime later its HorizontalContentAlignment is changed to Stretch. In that case, the content's size will change, but DropShadowPanel.SizeChanged might not fire.

It seems just listening to the SizeChanged event on the casting FrameworkElement is the right thing to do, since the shadow's dimensions need to be updated any time the casting element's dimensions change.

[nmetulev]

Hey @seanmcr, could you share a repro of this behavior?