Where two open paths lie along the same stretch, the planar map gave
that stretch to one of them only, so a merge or an erase across it left
the other path in place there. Every other open path along an edge now
has its own piece in `lines_under`, under a closed path's outline or
along a line in front. `lines`, `edges` and Live Paint's edges stay one
per stretch, and an Alt-click there still takes the piece in front.
Where a line lies under the outline of a closed path in front of it (such as
a shape an earlier merge made, which takes the place of the art in front of
the lines), the planar map gave that stretch only to the closed path, so the
line had no piece there. A merge or an erase across it left the line whole,
and the next map split the new shape along it into two regions.
- pathops: each planar edge records every input it lies on, and
BuilderArrangement::lines_under holds the lines' pieces under a closed
outline in front of them. lines, edges and Live Paint are unchanged.
- tools: BuilderMap adds those pieces to its lines, so a merge or an erase
removes the stretch between the regions it takes, as it does elsewhere.
An Alt-click where a line lies under an edge hits the one in front.
dotCooCoo