Introduce INIT-FREETYPE

I was recently bitten when trying to use cl-freetype2 in an image where it had been pre-loaded. My program promptly crashed with a less than helpful memory error. *LIBRARY* clearly must be re-initialized every time a lisp using cl-freetype2 is run. 

In order to make this fact more obvious and less clunky than asking the library user to call 
    
    (setf *library* (make-freetype))

this patch introduces a new function, INIT-FREETYPE, that does exactly this.