Fix order problem and missing deletes in OpenGL extension strings.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@4573 dfc29bdd-3216-0410-991c-e03cc46cb475
Reverting my changes from r4569 as they also break stuff by now. Will continue the discussion in http://irrlicht.sourceforge.net/forum/viewtopic.php?f=7&t=48940

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@4575 dfc29bdd-3216-0410-991c-e03cc46cb475