Add marker default line width test

Author: dy

test/image/mocks/gl2d_open_marker_line_width.json

@@ -0,0 +1,12 @@
+{
+  "data": [{
+    "y": [1, 2, 3],
+    "marker": {"symbol": "circle-open", "size": 20},
+    "type": "scattergl",
+    "mode": "markers"
+  }],
+  "layout": {
+    "height": 400,
+    "width": 480
+  }
+}