Physics: wat.

Brain fart in 39348ddf0633989aee8888e7dcfeff6b077e08ad (or probably f'ed
up merge conflict after crashed KDevelop while being interrupted for
lack of attention). Next time commit, stash and _then_ test.
SceneGraph: wrong assertion.

It is easier to just skip the object than explicitly removing
duplicates (or worse, cleaning it more than once).
Math: std::fma() is not available in NaCl's newlib.