ale.h: Add type 'ale_image_seq', as an image sequence type.
Such type could be used for supporting synchronization types (discussed in
ditz) as well as for replacing the ALE image_rw type, as this would allow for
things such as loading and caching strategies to be shared across interfaces.
It is expected that actual loading of images from the sequence (and
determination of length of the sequence) would occur via callback, as has
already been discussed in ditz for the case of synchronization types.