when generating a bitmap for a character whose codepoint was above 255,
we used a cache_id of 0, malloc'd a CachedImage and inserted it into a
g_hashtable, without freeing the one which was before inserted with cache_id 0
this is circumvented by only generating a CachedImage when having
a cache_id != 0
the second leak was also with inserting into a hashtable, but there we
give the hashtable the g_free method as a value_destroy_func