AFP2PDF Plus: Improve the caching of non MODCA image resources.

Currently AFP2PDF Plus caches non MODCA images (GIF, JPEG) after they have been processed and the data is ready to be used in the output PDF. This speeds up jobs that reuse these kinda of resources. However the current way of caching has no limit in term of how much will be cached. A job that uses a lot different image resources could fill the memory quickly.We need to implement a limit and a strategy of freeing up space if needed during a large job (for example removing ""least recently used"" resources from the cache).