Added generation of tangent space which can keep the original normals, by ryanclark. Note that inaccuracies of the tangent space may arise if you keep the original tangents. Angle weighted smoothing seems to be bogus, maybe indices are wrong.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@1392 dfc29bdd-3216-0410-991c-e03cc46cb475