[AutoLayout] Axis Alignment wrong for stretched non-filling elements

[carldebilly]

Considering the following XAML:

<utu:AutoLayout Background="Blue" Grid.Column="1" Spacing="5" PrimaryAxisAlignment="Stretch">
	<Rectangle Fill="Green" utu:AutoLayout.PrimaryAlignment="Stretch" utu:AutoLayout.CounterAlignment="Stretch" MaxWidth="100" MaxHeight="100" />
</utu:AutoLayout>

Current result

The rectangle will be aligned top-left in the AutoLayout

Expected

1. We would expect the rectangle to be centered both vertically and horizontally. It's not the case here.
2. Same result should apply for Width=100 and Height=100

Other

This Playground snippet also illustrate the problem: it should be centered.