add a few non-visible traces in gl2d mock

- to make sure they don't make Plotly.plot crash

Author: etpinard

test/image/mocks/gl2d_scatter-marker-line-colorscales.json

@@ -176,6 +176,22 @@
                 }
             },
             "uid": "0a4533"
+        },
+        {
+            "type": "scattergl",
+            "x": [],
+            "y": []
+        },
+        {
+            "type": "scattergl",
+            "y": []
+        },
+        {
+            "type": "scattergl",
+            "x": []
+        },
+        {
+            "type": "scattergl"
         }
     ],
     "layout": {
@@ -201,4 +217,4 @@
         "autosize": true,
         "showlegend": false
     }
-}
+}