Scale the sky box to the average of the near and far clip planes so that it doesn't get clipped.  Test by adding (e.g.) camera->setNearValue(100.f); to example 12.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@1807 dfc29bdd-3216-0410-991c-e03cc46cb475