[upstream] Simplified island merging, revised motor joint (box2d #966)#44

ikpil merged 2 commits intomainikpil/Box2D.NET:mainfrom

pr/Cull-back-side-shape-casts-against-chain-shapesikpil/Box2D.NET:pr/Cull-back-side-shape-casts-against-chain-shapesCopy head branch name to clipboard