add test to toself_tonext with >2 segments

Author: ruijin

test/image/mocks/gl2d_scatter_fill_self_next.json

@@ -17,7 +17,29 @@
             "y": [5, 6, 5],
             "fill": "toself",
             "type": "scattergl"
-        }
+        },
+        {
+          "x": [1.1, 1.1, 2.1, null,
+                -1.9,-1.9,-0.9,null,
+                -4.9,-4.9,-3.9],
+          "y": [6.1, 7.1, 7.1, null, 
+                1.1, 2.1, 2.1, null, 
+                1.1, 2.1, 2.1],
+          "type": "scattergl",
+          "fill": "tonext"
+      },
+      {
+          "x": [ 0, 0, 2, 2, 0, null,
+              1,1,3,3,1,null,
+              -2,-2,0,0,-2,null,
+              -5,-5,-3,-3,-5],
+          "y": [9, 10, 10, 9, 9, null,
+                6,8, 8, 6,6,null,
+                1,3,3,1, 1,null,
+                1,3,3,1,1],
+            "type": "scattergl",
+            "fill": "tonext"
+      }
     ],
     "layout":{
         "title": {"text": "Fill toself and tonext"},