Move visual DOM manipulation into RenderShadow...

...because only the shadow renderer needs to be able to do this.
Merge pull request #25 from arv/refactor-parallel-trees

Move visual DOM manipulation into RenderShadow...
Make offset*, client* and scroll* first flush the pending render queue
Merge pull request #26 from arv/implicit-render-all-pending

Make offset*, client* and scroll* first flush the pending render queue