[Skia.WPF] improve OnRenderSizeChanged()

[swharden]

This PR fixes a bug that causes the painted area not to match the window when resizing. If the OnRenderSizeChanged() override does not call its base implementation the SizeChanged event isn't raised and the dirty rectangle may be incorrectly sized when OnPaintSurface() is called. Note mono / SKElement.cs#L88-L93

WDSkiaGraphicsView inherits SKElement which inherits FrameworkElement

According to FrameworkElement.OnRenderSizeChanged() documentation:

You may still override OnRenderSizeChanged() in derived classes (it is protected but not sealed). Always call the base implementation to preserve the [layout] behavior mentioned above, unless you have very specific reasons for disabling default WPF framework-level rendering behavior. Failing to raise the SizeChanged event will cause non-standard layout behavior if using the standard WPF framework-level layout system implementation.

I realize this project is experimental/preview, so clarification on whether contributions are welcome at this time and if PRs must be accompanied by linked issues would be great! (#88)

Demonstrating the Issue

<skia:WDSkiaGraphicsView x:Name="GraphicsView" />

public partial class SandboxWindow : Window
{
    public SandboxWindow()
    {
        InitializeComponent();
        GraphicsView.Drawable = new BigX();
    }

    private class BigX : IDrawable
    {
        public void Draw(ICanvas canvas, RectangleF dirtyRect)
        {
            canvas.FillColor = Microsoft.Maui.Graphics.Colors.Navy;
            canvas.FillRectangle(dirtyRect);

            canvas.StrokeColor = Microsoft.Maui.Graphics.Colors.Yellow;
            canvas.DrawRectangle(dirtyRect);
            canvas.DrawLine(0, 0, dirtyRect.Width, dirtyRect.Height);
            canvas.DrawLine(dirtyRect.Width, 0, 0, dirtyRect.Height);
        }
    }
}

Before the Fix

Notice the white border around the filled area when resizing quickly

After the Fix

Filled area exactly matches the window size