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Flaw in LayoutSubviews/TopologicalSort

See original GitHub issue

Hello,

I just realized that the code in LayoutSubviews that collects the dependencies is incomplete:

                        foreach (var v in InternalSubviews) {
                                nodes.Add (v);
                                if (v.LayoutStyle == LayoutStyle.Computed) {
                                        if (v.X is Pos.PosView vX)
                                                edges.Add ((vX.Target, v));
                                        if (v.Y is Pos.PosView vY)
                                                edges.Add ((vY.Target, v));
                                        if (v.Width is Dim.DimView vWidth)
                                                edges.Add ((vWidth.Target, v));
                                        if (v.Height is Dim.DimView vHeight)
                                                edges.Add ((vHeight.Target, v));
                                }
                        }

The issue is that this is not taking into account the Dim and Pos subclasses that combine two views, so we only do the proper job of topologically sorting when there is a direct dependency, not when there is something like view + 1 (when we create a combined pos/dim)

The code needs to handle PosCombine/DimCombine and recursively add the elements to it.