Every flattened segment scanned its box grown by the largest feather radius,
so a path with many vertices and a radius spanning the mask revisited every
pixel once per segment. The pixel visits are now counted first; past a
budget of 2^31 the flattened polyline is thinned (every other point, arc
lengths kept) until it fits, and only a degenerate case falls back to the
hard mask.
Dylan M. Taylor
fd10a4f326