closes #2873 - add line colorscale + markers mock

... locking down the fix for #2873 completed in
    c9d92d7

Author: etpinard

test/image/baselines/gl3d_line-colorscale-with-markers.png

@@ -0,0 +1,15 @@
+{
+  "data": [
+    {
+      "type": "scatter3d",
+      "x": [0, 0, 0],
+      "y": [1, 1, 1],
+      "z": [1, 2, 3],
+      "mode": "lines+markers",
+      "line": {
+        "color": [0, 1, 2],
+        "width": 10
+      }
+    }
+  ]
+}