changed CAnimatedMeshSceneNode to use the bounding box of its IAnimatedMesh instead of calling getMesh() and re-skinning. Tested with a few different mesh formats and couldn't see any obvious problems. IAnimatedMesh implementations are now responsible for keeping a maximum bounding box.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@704 dfc29bdd-3216-0410-991c-e03cc46cb475