This involves reimplementing QWidgetPrivate::grabFramebuffer().
Widgets call this function whenever a texture-based widget is
encountered.
This implies however that we rename QRhiWidget's own, lightweight
grab function, grab(), because it kind of shadows QWidget's grab().
Switch back to grabFramebuffer() which is what QQuickWidget and
QOpenGLWidget both use.
Supporting QWidget::grab() is particularly important when grabbing
an ancestor of the QRhiWidget, because that has no alternative.
Right now, due to not reimplementing the QWidgetPrivate function,
the place of the QRhiWidget is left empty.
In addition, grabFramebuffer() is now const. This is consistent
with QQuickWidget, but not with QOpenGLWidget and QOpenGLWindow.
Change-Id: I646bd920dab7ba50415dd7ee6b63a209f5673e8f
Reviewed-by: Andy Nichols <andy.nichols@qt.io>
