Add complex test

Author: JimBobSquarePants

tests/ImageSharp.Drawing.Tests/Drawing/Paths/ComputeLength.cs

@@ -14,5 +14,15 @@ public void CanComputeUnrolledLength()
 
             Assert.Equal(600, polygon.ComputeLength());
         }
+
+        [Fact]
+        public void CanComputeUnrolledLengthComplexPath()
+        {
+            var polygon = new ComplexPolygon(
+                new RectangularPolygon(PointF.Empty, new PointF(100, 200)),
+                new RectangularPolygon(new PointF(1000, 1000), new PointF(1100, 1200)));
+
+            Assert.Equal(1200, polygon.ComputeLength());
+        }
     }
 }