Fix ServerCompositionSimplePen memory leak

[aguahombre]

What does the pull request do?

Bug fix for the ServerCompositionSimplePen memory leak

What is the current behavior?

ServerCompositionSimplePen objects are leaking as when the compositor creates ServerCompositionSimplePen resources that use shared brushes defined in the Theme or Window resources.

What is the updated/expected behavior with this PR?

Fixes the memory leak.

How was the solution implemented (if it's not obvious)?

The ServerCompositionSimplePen is being held in memory because it it is added to the ServerCompositionSimpleBrush list of observers but is never removed when the pen is disposed.
If the associated brush is long lived; i.e. a static or dynamic resource created in the theme or main window, the number of ServerCompositionSimplePen in it's list of observers increases every time a pen is created for that brush.
Added a Dispose override to remove the pen from the brush observers.

Checklist

Not sure how to add a unit test for this as it doesn't appear to be possible to get a count of live objects of a class from GC.

Breaking changes

None

Obsoletions / Deprecations

None

Fixed issues

Fixes #16451

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0059690-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

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[MrJul]

The property change will cause the pen to be queued for invalidation, which isn't needed since the object is being disposed. I think a better fix would be to call RemoveObserversFromProperty(ref _brush).

[aguahombre]

Yes, that makes sense, I'll make the change.
SetValue nulls the brush, so I'll set the brush to null, although it should not be necessary if the pen get GCed.

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0059790-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]