add support for multi-frame images via imlib2 (#373)

this will be a massive change compared to the usual stuff. however the
gains will be worth it:

* we gain lots of additional animated image support.
* and we'll gain _even_ more format support as imlib2 adds them, without needing
  any change in our code-base.
* about ~300 LoC will be purged once we remove our internal gif and webp loader.

as for when to remove the internal loaders, a good time might be when debian
upgrades their imlib2, currently it seems to be at v1.7.5, which doesn't support
animated images.

as of now, nsxiv will continue to build with the internal gif/webp loaders
(assuming they were enabled in config.mk) if imlib2 version is below 1.8.0 and
will print out a deprecation notice.

and if imlib2 version supports multi-frame then it will simply ignore the
internal loaders and use the imlib2 one.

in other words, users shouldn't need to do anything on their side. everything
that previously functioned will continue to function regardless of the user's
imlib2 version (though they might see the annoying deprecation notice if the
imlib2 version doesn't support multi-frame images).

known issue:

* image loading performance can be noticeably worse in
  imlib2 versions below 1.9.0

Closes: https://codeberg.org/nsxiv/nsxiv/issues/301
Closes: https://codeberg.org/nsxiv/nsxiv/issues/300
Reviewed-on: https://codeberg.org/nsxiv/nsxiv/pulls/373
Reviewed-by: TAAPArthur <taaparthur@noreply.codeberg.org>

1 files changed, 4 insertions, 1 deletions

diff --git a/config.mk b/config.mk
index 10fd491..b79a63a 100644
--- a/config.mk
+++ b/config.mk
@@ -14,8 +14,11 @@ HAVE_INOTIFY = $(OPT_DEP_DEFAULT)
 
 # optional dependencies, see README for more info
 HAVE_LIBFONTS = $(OPT_DEP_DEFAULT)
-HAVE_LIBGIF   = $(OPT_DEP_DEFAULT)
 HAVE_LIBEXIF  = $(OPT_DEP_DEFAULT)
+
+# unused if imlib2 version is 1.8.0 or higher.
+# these options will be removed eventually.
+HAVE_LIBGIF   = $(OPT_DEP_DEFAULT)
 HAVE_LIBWEBP  = $(OPT_DEP_DEFAULT)
 
 # Compiler and linker