Re: trunk r113947: * image.c: Fix animation cache signature memory leak.

[Lars Magne Ingebrigtsen <larsi@gnus.org>]

I think what might really help with the speed is doing more caching and
a better liveliness analysis.  Or something.

First of all, every iteration we re-parse the "super-wand":

  if (filename)
    status = MagickReadImage (image_wand, filename);
  else
    {
      filename_hint = imagemagick_filename_hint (img->spec, hint_buffer);
      MagickSetFilename (image_wand, filename_hint);
      status = MagickReadImageBlob (image_wand, contents, size);
    }

If we stash that in the cache, too, then my guess would be that we'd see
a rather large speed-up.  We'd have to have a way to compute an ID for
the data, though.  I'd suggest just doing a sha256 or something over
contents and then perhaps using the same animation cache.  It'd need
some tweaks, though, because we don't want to cache non-animation
images, and we don't know whether they are or not before we've parsed
it.  :-)

The other thing is that imagemagick_compute_animated_image clones the
wand before putting it into the cache:

  cache->wand = CloneMagickWand (composite_wand)

This is because imagemagick_load_image later destroys the wand, so we
need a copy.  This could be avoided, of course, and my guess is that
this would also be a nice speed-up, because copying a wand must probably
entail copying the entire data.

If you could look at this, I think the ImageMagick animation could start
approaching the gif animation code speed-wise.  At present, it's much
slower, especially on large images.

This is the code snippet I use to test:

(url-retrieve "http://cdn.arwrath.com/1/148330.gif"
	      (lambda (&rest ignore)
		(search-forward "\n\n")
		(let ((image (create-image
			      (buffer-substring (point) (point-max))
			      'imagemagick t)))
		  (pop-to-buffer "*image*")
		  (erase-buffer)
		  (delete-all-overlays)
		  (put-image image (point) "*")
		  (image-animate image nil 60))))


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