Use a Clip rect for Pane animations instead of the Width/Height

follow up from #7364

Looking at this doc:

An animation is independent if it has any of these characteristics:

• The Duration of the animation is 0 seconds (see Warning)
• The animation targets UIElement.Opacity
• The animation targets a sub-property value of these UIElement properties: Transform3D, RenderTransform, Projection, Clip
• The animation targets Canvas.Left or Canvas.Top
• The animation targets a Brush value and uses a SolidColorBrush, animating its Color
• The animation is an ObjectAnimationUsingKeyFrames

emph added

Right now we're stuck being a dependent animation, because we're animating the Width or Height. But we might be able to use Clip to achieve the same effect.

I'm no XAML wizard, so that'll require another day of learning to figure out, but I figured I'd write these notes down before I forgot.

Might even be able to get us #7365 for free.