We remember the location of the last insert as the next edge is likely to
be nearby. However, we need to be careful when the pointer rests upon the
HEAD and ensure that we begin the search from the appropriate end.
Add an even simpler sweep-line tessellator for rectangular trapezoids (as
produced by the rectilinear stoker and box filler).
This is so simple it even outperforms pixman's region validation code for the
purposes of path-to-region conversion.
