bug#12463: 24.2; pos-visible-in-window-p gets slower over time

[Eli Zaretskii]

> From: Juanma Barranquero <lekktu@gmail.com>
> Date: Sat, 22 Sep 2012 21:20:48 +0200
> Cc: Eli Zaretskii <eliz@gnu.org>, jwalt@garni.ch, 12463@debbugs.gnu.org
> 
> Eli and I had talked about forcing the ops that work on Vlibrary_cache
> and image_types to not cache anything before dumping, but now that the
> entries for xbm and pbm are added the same way that other image
> libraries, I wonder whether wouldn't be easier/cleaner to simply set
> Vlibrary_cache = Qnil and image_types = NULL just before dumping.

I think that would leak memory.  Not a catastrophe, but still not
clean.