Add flag to avoid duplicated skinning in one frame for the same mesh and animation frame. Needs to be checked if all cases where a setting has changed and skinning needs to be applied, has been properly marked and resets the flag.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@3934 dfc29bdd-3216-0410-991c-e03cc46cb475