Function contourToPolygon

Attempts to attach a convex b2PolygonShape to bodyId.

Returns the new shape id on success, or null if the contour is not eligible. Eligibility:

• At least 3 vertices.
• At most 8 vertices (Box2D's b2PolygonShape cap).
• Convex (all consecutive cross products have the same sign).

Callers (typically the debris path of Box2DAdapter) should fall back to contourToChain when this returns null.