Region.bounding_box: Remove limit of just passing two regions
Region.bounding_box: Remove limit of just passing two regions

`bounding_box` is associative and commutative so there's no reason
not to.

stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester