Add fill self next test

Author: dy

Diff for: test/image/mocks/gl2d_scatter_fill_self_next.json

@@ -0,0 +1,28 @@
+{
+    "data":[
+        {
+            "x": [1, 2, 3, 1, null, 4, 5, 6],
+            "y": [2, 3, 2, 2, null, 3, 4, 3],
+            "fill": "tonext",
+            "line":{"shape": "spline"},
+            "type": "scattergl"
+        },
+        {
+            "x": [-1, 4, 9, null, 0, 1, 2],
+            "y": [1, 6, 1, null, 5, 6, 5],
+            "fill": "tonext",
+            "type": "scattergl"
+        },
+        {
+            "x": [6, 7, 8],
+            "y": [5, 6, 5],
+            "fill": "toself",
+            "type": "scattergl"
+        }
+    ],
+    "layout":{
+        "title": "Fill toself and tonext",
+        "width": 400,
+        "height": 400
+    }
+}