Graphics.FillPolygon can crash on malformed point arrays

Graphics.FillPolygon and Graphics.DrawPolygon expected a flat Array(Of Double) containing x/y coordinate pairs, but malformed arrays could reach the desktop graphics native method and throw an internal InvalidCastException instead of a friendly Objo runtime error. Polygon drawing now validates the input and reports InvalidArgument errors for non-array, nested, too-short, odd-length, or non-numeric point data.