Add a generic attribute interface for querying video driver attributes which are not necessarily of type bool. This interface allows to check certain supported features, such as the number of user clip planes, supported lights and textures, MRTs, and other things. The interface might change in the future, but it's fully functional already. The supported attributes are listed in the API docs of the function.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@3408 dfc29bdd-3216-0410-991c-e03cc46cb475