Hi,

I'm trying to revisit this idea of having a user controlled image cache.
The goal is to have, for instance, image-mode use it because (hopefully)
it would have a better idea of when to cache/uncache images for its
usage.

Here is a patch of such a prototype « user image cache ».

For the moment, I wanted to keep it minimal so the only interface to use
it goes as follow.  When a user (or a program) use the image attribute
":ttl TIME_IN_SECONDS" with `create-image' such image will be stored in
the « user cache » and not in the internal one.  Images lookups use both
caches (internal and internal, in that order).  A negative
TIME_IN_SECONDS means « cache this image forever ».

I think there will be a need for user function that completely flushes
the images in this new cache.  But do you think we need others commands
for such interface?