Fix broken zoom functionality (danvk#611) in the drawing test

Same as PR#953 fixed for the gallery, really.

tests/drawing.html

@@ -143,6 +143,9 @@ <h2>Time Series Drawing Demo</h2>
           labels: [ 'Date', 'Value' ],
           labelsUTC: true,
           interactionModel: {
+            // the next line is required when using the
+            // defaultInteractionModel mousedown function.
+            willDestroyContextMyself: true,
             mousedown: function (event, g, context) {
               if (tool == 'zoom') {
                 Dygraph.defaultInteractionModel.mousedown(event, g, context);
@@ -153,21 +156,25 @@ <h2>Time Series Drawing Demo</h2>
               }
             },
             mousemove: function (event, g, context) {
-              if (tool == 'zoom') {
-              } else {
+              // note that the defaultInteractionModel dynamically binds
+              // its own mousemove event inside the mousedown handler
+              if (tool != 'zoom') {
                 if (!isDrawing) return;
                 setPoint(event, g, context);
               }
             },
             mouseup: function(event, g, context) {
-              if (tool == 'zoom') {
-              } else {
+              // note that the defaultInteractionModel dynamically binds
+              // its own mouseup event inside the mousedown handler
+              if (tool != 'zoom') {
                 finishDraw();
               }
             },
             mouseout: function(event, g, context) {
-              if (tool == 'zoom') {
-              }
+              // note that the defaultInteractionModel does not use
+              // the mouseout event, instead it detects when the mouse
+              // is outside the chart using a dynamically bound
+              // mousemove event
             },
             dblclick: function(event, g, context) {
               Dygraph.defaultInteractionModel.dblclick(event, g, context);