Bugfix: IMeshManipulater functions recalculateNormals and recalculateTangents no longer create nan's 
Was caused by getAngleWeight function passing values beyond [-1, 1] range to acosf

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@6678 dfc29bdd-3216-0410-991c-e03cc46cb475