Hi Eli and Stefan,

Here is a new version for my patch to add some user control on the image
cache.  This time, I have done some tests and benchmarks.

Instead of trying to have a completely different cache, I'm using the
current one but I had a condition to flush (or not) an image based on a
user defined time to live (TTL) for an image.  This TTL is a number of
seconds since the image was last displayed.

I have also tested it in image-mode with the second patch.  I tested by
browsing a directory with some rather large pictures in it (4000x3000
pixels, ≈6MB for each file).  I opened the first image in a 1640x1000
window (so each image is scaled) and did the following:

  - M-: (dotimes (i 10) (image-next-file 1))  ;; for cache warming
  - M-: (dotimes (i 10) (image-next-file -1)) ;; return to first image
  - M-: (benchmark-run 10 (dotimes (i 10) (image-next-file 1))
                          (dotimes (i 10) (image-next-file -1)))

Here is the timings I get:

     - with master (ea685170063): (109.208767158 15 1.0938777159999997)
     - with this patch: (6.934307561 15 1.095243891)

WDYT?