broadway: Prune fully clipped render nodes

broadway: Prune fully clipped render nodes

If some node is fully outside the clip region we don't send it to the daemon.
This helps a lot in how much data we send for scrolling viewports.

However, sending partial trees makes node reuse a bit more tricky. We
can't save for reuse any node that could possibly clip different depending on
the clip region, as that could be different next frame. So, unless the
node is fully contained in the current clip (and we thus know it is not
parial) we don't allow reusing that next frame.

This fixes #3086