QVTKOpenGLWidget: better paintGL/Render interaction.

`QVTKOpenGLWidget::paintGL` no longer results in a `Render` call on
every paintGL call. It only does that for cases where it's an absolute
must e.g. paintGL after recreating FBO. QVTKOpenGLWidget now relies on
VTK applications calling `Render` on the window when data or rendering
has changed.

For those few times where QVTKOpenGLWidget has to call Render in
`paintGL`, it does that via a new virtual method `renderVTK`.

This also removes deferred-rendering API. That API was only added to
attempt an API match with QVTKWidget. Since that is already not true, it
makes sense to keep this new class as simple as possible. Besides, the
new `renderVTK` API makes it possible for subclasses to implement
deferred-rendering support, if needed.