RelativePanel doesnt measure correctly when items are relative to each other and panel isnt stretched

[danwalmsley]

Describe the bug
A clear and concise description of what the bug is.

<RelativePanel Background="White" VerticalAlignment="Top" HorizontalAlignment="Left">
            <Rectangle Name="rect1" Height="20" Width="20" Fill="Red"  />
            <Rectangle Name="rect2" Height="10" Width="10" Fill="Yellow" RelativePanel.Below="rect1" />
 </RelativePanel>

I expect:

however.... (provided cliptobounds=true)

I actually get...

This is because the measure is just looking for the largest object and not taking into account the positioning.

Probably needs to use the graph to measure also.

To Reproduce
Steps to reproduce the behavior:

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

Expected behavior
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Screenshots
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Environment (please complete the following information):

• .net: [e.g. 4.6.2]
• IDE [e.g. vs2017]
• Version [e.g. 1.0.0]

Additional context
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[NaBian]

fixed, but more tests may be needed