Fix bug with vectorscope not updating

The scope would not redraw after switching from one vectorscope to
another. This commit sets the drawing area dirty when the scope type
changes.

rtgui/histogrampanel.cc

@@ -497,6 +497,8 @@ void HistogramPanel::type_selected(Gtk::RadioButton* button)
     if (histogramRGBArea) {
         updateHistRGBAreaOptions();
     }
+    histogramArea->setDirty(true);
+    histogramArea->queue_draw();
 }
 
 void HistogramPanel::type_changed()