Add parameter useAlphaChannel to second IGUIEnvironment::addImage function as it's rather confusing when they have different defaults. Unfortunately can't use same parameter order anymore without messing up existing user-code now (some things can only be done right on first try...).

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@4223 dfc29bdd-3216-0410-991c-e03cc46cb475