Fix QWidgetBackingStore::isDirty() for render-to-texture widgets

The backing store would not report itself dirty if only render-to-
texture widgets were dirty.

This caused QOpenGLWidgets not be repainted after being remapped if they
marked themselves dirty while unmapped.

Task-number: QTBUG-50818
Task-number: QTBUG-51815
Change-Id: If43f7cbb6f4d44831b4c21b89f884d6bcaebf87c
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>