gl2d: add user-select-none class to canvas

- this sanitizes double clicks interactions
- e.g. double-click on the canvas now no longer puts
  the select-box in a weird state
- see #744 for more info

src/plots/gl2d/scene2d.js

@@ -137,6 +137,11 @@ proto.makeFramework = function() {
     canvas.style.left = '0px';
     canvas.style['pointer-events'] = 'none';
 
+    // disabling user select on the canvas
+    // sanitizes double-clicks interactions
+    // ref: https://github.com/plotly/plotly.js/issues/744
+    canvas.className += 'user-select-none';
+
     // create SVG container for hover text
     var svgContainer = this.svgContainer = document.createElementNS(
         'http://www.w3.org/2000/svg',