Fix rendering with only index buffers. Basically, this is now removed, because d3d seems not to support this. Hence we can only render with hw buffers, if both vertex and index buffers are enabled.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@3908 dfc29bdd-3216-0410-991c-e03cc46cb475