Quick reference guide for using the imagecache:
- Access: pc::imgcache.
- Loading: pc::imgcache->loadImage(filename);  Returns number required for image
  retrieval.  Throws ImageNotFound on error.
- Getting images:  pc::imgcache->getImage(num);  Where num is (number returned
  by loadImage)+(frame offset) | (pallete << 11).  Returns an ImageCacheEntry
  struct (defined in imagecache.h).
- ImageCacheEntry: Two data members: image and shadow (both SDL_Surface*).
  (surfaces are owned by the imagecache, so don't use SDL_FreeSurface)
