getStrokeOutlinePoints

Walk the stroke points and produce the polygon that surrounds them.

At each point the outline steps out perpendicular to the direction of travel by that point's radius, once to the left and once to the right; the right side is walked back in reverse so the result is a single closed ring. Tapers shrink the radius near the ends, and a cap fans extra points around the terminal so it reads as round rather than cut off.