MaximalRectangles.InRegion finds the largest axis-aligned rectangles that fit
wholly inside a Clipper region, such as a cutout shrunk by the part spacing.
It grids the region on its vertex coordinates plus even divisions, keeps a cell
only when no edge enters it and an even-odd row scan puts it inside, then runs
the shared histogram search.
Exact for regions with only horizontal and vertical edges; slanted and curved
edges are followed as a staircase that never crosses the boundary, short of the
true maximum by up to about one cell per side. Tests cover a rectangle, an
L shape, a frame with a hole, a round hole (inscribed square), a diamond and a
star; disabling the edge-crossing check fails the three slanted-edge tests.
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