Prepare to make RenderObjectElement buildable

This patch prepares us to pass a BuildContext to RenderObjectWidgets, which
will make it possible to rebuild RenderObjectElements:

 * Delay creation of the render object until mount(). That will let us pass
   `this` to createRenderObject and have the inherited elements be initialized.

 * Cleanup widgets that take builder closures to prepare for their
   RenderObjectElement to be rebuilt more often.

 * Add a test for the interaction between inherited widgets and
   MixedViewport.

Related to #2598