Fix regression when using Windows XP with QVTKWidget.

When another app window is passing over a QVTKWidget, there was some
uncessary flicker on Windows XP.  The flicker was caused by an
extra InvalidateRect() call that we actually need for Vista and newer.

Change-Id: I37df2b4b66c06779c8e949d6993d53c41d2c8355
Merge topic 'qvtk-aero-xp' into master

ce1e7cdb Fix regression when using Windows XP with QVTKWidget.