Viewbox scaling fix for VLC

[RobG66]

I came up with a very simple, but effective fix for libvlcsharp.avalonia not scaling in a viewbox in Avalonia. You put the videoview inside a viewbox as you normally would and subscribe to bounds changes on the viewbox:

_boundsSubscription = _viewbox.GetObservable(Viewbox.BoundsProperty)
    .Subscribe(newBounds => OnViewboxBoundsChanged(newBounds));

private void OnViewboxBoundsChanged(Rect newBounds)
{
    if (_viewbox == null || _videoView == null)
        return;

    var newSize = newBounds.Size;
    if (newSize == _lastViewboxSize || newSize.Width <= 0 || newSize.Height <= 0)
        return;

    _lastViewboxSize = newSize;

    // Calculate the scaled size of the VideoView within the Viewbox
    // Viewbox uses Uniform stretch, so we need to calculate aspect-ratio-preserving size
    const double videoWidth = 400;
    const double videoHeight = 300;
    var videoAspect = videoWidth / videoHeight;  // 4:3 ratio
    var viewboxAspect = newSize.Width / newSize.Height;

    double scaledWidth, scaledHeight;

    if (viewboxAspect > videoAspect)
    {
        // Viewbox is wider - constrain by height
        scaledHeight = newSize.Height;
        scaledWidth = scaledHeight * videoAspect;
    }
    else
    {
        // Viewbox is taller - constrain by width
        scaledWidth = newSize.Width;
        scaledHeight = scaledWidth / videoAspect;
    }

    var newWidth = Math.Max(1, scaledWidth);
    var newHeight = Math.Max(1, scaledHeight);

    // Update VideoView size to match the scaled size
    _videoView.Width = newWidth;
    _videoView.Height = newHeight;
    
    // Force layout update to ensure native window resizes
    _videoView.InvalidateMeasure();
    _videoView.InvalidateArrange();
}