Bugfix change device source resolution

DeviceSource.cpp

@@ -384,10 +384,13 @@ void DeviceSource::setDevice(const std::string &devicename)
 
         pipeline << " ! videoconvert";
 
+        dettach();
+
         // open gstreamer
         stream_->open( pipeline.str(), best.width, best.height);
         stream_->play(true);
 
+
         // will be ready after init and one frame rendered
         ready_ = false;
     }