This allows the painter to render filled complex shapes better, by
constructing a path graph for (interesting) intersecting lines and
omitting lines from the containing segments if they are detected
to take no part in defining the edges of a shape.
This approach would still fail if there are multiple logical shapes
that are confined to the collection of lines.
For instance, two polygons intersecting each other in a way that one
vertex of polygon A ends up inside polygon B.
we would detect that polygon A's edges are part of the shape
(technically correct) even though they are not a part of polygon B at
all.
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