Allow clients to know when a render occurred that was not stale.

Change-Id: Ia3f3d4ac9e970f0827321329a9ac341f897453d2

Web/JavaScript/Lib/vtkweb.viewport.image.js

@@ -177,6 +177,10 @@
                     // request another render to try to get the latest image.
                     if (res.stale === true) {
                         renderOnIdle();
+                    } else {
+                        container.trigger({
+                            type: 'renderer-ready'
+                        });
                     }
                 });
             }

Web/JavaScript/Lib/vtkweb.viewport.js

@@ -537,6 +537,18 @@
             throw "'session' must be provided within the option.";
         }
 
+        function onReadyInternal() {
+            if(userReadyCallback.fn) {
+                // Providing just a short delay before triggering the "image-not-state"
+                // event provides a break in the action so that the browser can draw the
+                // image we just copied to the src attribute.
+                setTimeout(userReadyCallback.fn, 0);
+                if(userReadyCallback.once) {
+                    userReadyCallback.fn = null;
+                }
+            }
+        }
+
         // Create viewport
         var config = $.extend({}, DEFAULT_VIEWPORT_OPTIONS, options),
         session = options.session,
@@ -547,6 +559,7 @@
         onDoneQueue = [],
         statisticManager = createStatisticManager(),
         inSelectionMode = false,
+        userReadyCallback = { fn: null, once: false},
         viewport = {
             /**
              * Update the active renderer to be something else.
@@ -669,6 +682,14 @@
                 rendererContainer.trigger('clearCache');
             },
 
+            /*
+             * Provide a function that will be called when
+             */
+            onReady: function(readyCallback, once) {
+                userReadyCallback.fn = readyCallback;
+                userReadyCallback.once = once;
+            },
+
             /**
              * Attach viewport to a DOM element
              *
@@ -802,6 +823,9 @@
             }
         });
 
+        // Attach the on ready internal
+        rendererContainer.bind('renderer-ready', onReadyInternal);
+
         // Create any renderer type that is available
         for(var key in vtkWeb.ViewportFactory) {
             try {