In $draw exit if mode!=Canvas instead of mode==DOM

In default $draw code, exit if current mode is not
QMLRenderMode.Canvas. This is better than check that mode not equal to
DOM. Because we can have some new modes, for example "zero"-mode when
we do not need rendering at all. This is useful for serverside or
scripting scenarios.

src/qtcore.js

@@ -809,7 +809,7 @@ QMLEngine = function (element, options) {
 
     this.$draw = function()
     {
-        if (this.renderMode == QMLRenderMode.DOM)
+        if (this.renderMode != QMLRenderMode.Canvas)
             return;
         var time = new Date();