Fix interaction/image tests

Author: dy

src/plots/cartesian/dragbox.js

@@ -715,9 +715,11 @@ module.exports = function dragBox(gd, plotinfo, x, y, w, h, ns, ew) {
         // FIXME: code duplication with cartesian.plot
         if(fullLayout._glcanvas && fullLayout._glcanvas.size()) {
             fullLayout._glcanvas.each(function(d) {
-                d.regl.clear({
-                    color: true
-                });
+                if (d.regl) {
+                    d.regl.clear({
+                        color: true
+                    });
+                }
             });
         }
 

src/plots/cartesian/index.js

@@ -51,9 +51,11 @@ exports.plot = function(gd, traces, transitionOpts, makeOnCompleteCallback) {
     // clear gl frame, if any, since we preserve drawing buffer
     if(fullLayout._glcanvas && fullLayout._glcanvas.size()) {
         fullLayout._glcanvas.each(function(d) {
-            d.regl.clear({
-                color: true
-            });
+            if (d.regl)  {
+                d.regl.clear({
+                    color: true
+                });
+            }
         });
     }