Improve spritebank slightly (more comfort functions and documentation).


git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@4751 dfc29bdd-3216-0410-991c-e03cc46cb475
Mention in documentation that getSpriteBank currently doesn't load files (that's never been implemented).


git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@4752 dfc29bdd-3216-0410-991c-e03cc46cb475