This reverts commit 8a679e20.
On Windows, Qt::WA_PaintOnScreen is actually supported when providing a
paintEngine. This is what QGLWidget does, even on Windows.
The problem we faced with "Fix QVTKWidget refresh on resize problem on
Windows" was that when using the cache image, the qt backingstore was
flushing (on window activated event) an old buffer of the qvtkwidget instead
of the cached image.