Memory leak fix candidates: 1) purge must destroy scene2d otherwise rAF never stops; 2) scene2d.destroy should destroy the traces

(cherry picked from commit 8f6f2dd)

Author: monfera

src/plots/gl2d/scene2d.js

@@ -318,6 +318,14 @@ proto.destroy = function() {
 
     this.glplot = null;
     this.stopped = true;
+
+    var traces = this.traces
+    if(traces) {
+        Object.keys(traces).map(function(key) {
+            traces[key].dispose();
+            traces[key] = null;
+        })
+    }
 };
 
 proto.plot = function(fullData, calcData, fullLayout) {

src/plots/plots.js

@@ -809,6 +809,16 @@ plots.purge = function(gd) {
     // remove modebar
     if(fullLayout._modeBar) fullLayout._modeBar.destroy();
 
+    if(fullLayout._plots) {
+        Object.keys(fullLayout._plots).map(function(key) {
+            var plot = fullLayout._plots[key];
+            if(plot._scene2d) {
+                plot._scene2d.destroy();
+                plot._scene2d = null;
+            }
+        });
+    }
+
     // data and layout
     delete gd.data;
     delete gd.layout;