Fix cache reset and expired pixbuf cache (see issue #1179)

Only reset pixbuf cache entries that are older than the reset.  This prevents the entire cache being discarded the first time this is called because artwork_reset_time is zero.
Check pixbuf modification time against disk cache modification time to pick up fresh artwork.  Would be nice to get mtime from get_album_art() and avoid the extra stat(), but I don't see how without changing the API.