fixing BoundingBox mul

multiplication on bounding box must be set to inf() before using ext().
because a transformation like a translation is not an extension of the bounding box, so ext() is not valid in this case.
boundingbox must be reset with inf() first.
Merge pull request #2469 from titoasty/master

fixing BoundingBox mul
Merge branch 'master' of https://github.com/libgdx/libgdx into netstuff
Added note to CHANGES.