add gl2d line opacity test case

Author: etpinard

test/image/mocks/gl2d_line_opacity.json

@@ -0,0 +1,44 @@
+{
+  "data": [
+    {
+      "x": [
+        1,
+        2,
+        null,
+        4,
+        5
+      ],
+      "y": [
+        1,
+        2,
+        null,
+        4,
+        5
+      ],
+      "connectgaps": false,
+      "type": "scattergl",
+      "opacity": 0.4
+    }
+  ],
+  "layout": {
+    "xaxis": {
+      "type": "linear",
+      "range": [
+        0.9867209252516599,
+        5.01327907474834
+      ],
+      "autorange": true
+    },
+    "yaxis": {
+      "type": "linear",
+      "range": [
+        0.9529946929492039,
+        5.047005307050796
+      ],
+      "autorange": true
+    },
+    "height": 450,
+    "width": 1100,
+    "autosize": true
+  }
+}