wxCarioContext:Flush now draws back to the QT image

wxCairoContext::Flush was flushing back to the internal image but this
image wasn't drawn back to the QPainter until the wxCarioContext
instance was destroyed.

This fix ensure that after a call to Flush, anything drawn by Cario is
drawn back to the QImage.

Closes https://github.com/wxWidgets/wxWidgets/pull/1068

src/generic/graphicc.cpp

@@ -2566,6 +2566,7 @@ void wxCairoContext::Flush()
     if ( m_qtSurface )
     {
         cairo_surface_flush(m_qtSurface);
+        m_qtPainter->drawImage( 0,0, *m_qtImage );
     }
 #endif
 }