Add preliminary HEIF/AVIF support

The gdk-pixbuf plugin does not work here, for whatever reason.

Moreover, close integration exposes higher bit depths, metadata,
and auxiliary images.

The library is awful and copylefted, but it's the only reasonable
thing that works.

meson.build

@@ -17,6 +17,7 @@ endif
 libraw = dependency('libraw', required : get_option('libraw'))
 librsvg = dependency('librsvg-2.0', required : get_option('librsvg'))
 xcursor = dependency('xcursor', required : get_option('xcursor'))
+libheif = dependency('libheif', required : get_option('libheif'))
 libtiff = dependency('libtiff-4', required : get_option('libtiff'))
 gdkpixbuf = dependency('gdk-pixbuf-2.0', required : get_option('gdk-pixbuf'))
 dependencies = [
@@ -28,6 +29,7 @@ dependencies = [
 	libraw,
 	librsvg,
 	xcursor,
+	libheif,
 	libtiff,
 	gdkpixbuf,
 	meson.get_compiler('c').find_library('m', required : false),
@@ -39,6 +41,7 @@ conf.set_quoted('PROJECT_VERSION', meson.project_version())
 conf.set('HAVE_LIBRAW', libraw.found())
 conf.set('HAVE_LIBRSVG', librsvg.found())
 conf.set('HAVE_XCURSOR', xcursor.found())
+conf.set('HAVE_LIBHEIF', libheif.found())
 conf.set('HAVE_LIBTIFF', libtiff.found())
 conf.set('HAVE_GDKPIXBUF', gdkpixbuf.found())
 configure_file(