snapashot: Optimize rounded clip nodes

snapashot: Optimize rounded clip nodes

If the rounded clip node is rectilinear, we can simplify it to a normal
clip node. If not, we really need to use a rounded clip node. In both
cases, we can do the same check we do when collecting normal clips and
avoid the clip node altogether if the child node does not get clipped
anyway.

This saves between 3 and 10 nodes in the widget factory, depending on
what page gets rendered.