QmlProfiler: Try harder to obtain a canvas context

The QML canvas is notoriously bad at keeping its context around. This
workaround gives us a valid context in many of these cases.

Change-Id: I6c23aa2b44c10a48cdb89c66a2861ab8d31bcfc8
Reviewed-by: Kai Koehne <kai.koehne@digia.com>

src/plugins/qmlprofiler/qml/Overview.qml

@@ -112,8 +112,7 @@ Canvas {
 
     // ***** slots
     onPaint: {
-        if (context === null)
-            return; // canvas isn't ready
+        var context = (canvas.context === null) ? getContext("2d") : canvas.context;
 
         Plotter.qmlProfilerModelProxy = qmlProfilerModelProxy;