Revert "Fix QVTKWidget refresh on resize problem on Windows"

This reverts commit d09348ec9bb8324541fc9d7f8d784bacd8852429.
Disable double buffering on Windows

When switching back from a top level widget focus, the double buffering on
Windows draws an old and obsolete frame.
By disabling double buffering, the old frame is not painted.
However, this has the minor drawback of making the resize repaint
asynchronous.