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.

README.adoc

@@ -2,7 +2,7 @@ fastiv
 ======
 
 'fastiv' is a fast image viewer, supporting BMP, PNG, GIF, JPEG, and optionally
-raw photos, SVG, X11 cursors and TIFF, or whatever gdk-pixbuf loads.
+raw photos, HEIC, AVIF, SVG, X11 cursors and TIFF, or whatever gdk-pixbuf loads.
 
 It still has some road to go, but it's already become quite usable,
 and it has received basic polishing.
@@ -24,8 +24,9 @@ Building and Running
 --------------------
 Build dependencies: Meson, pkg-config +
 Runtime dependencies: gtk+-3.0, glib>=2.64, pixman-1, shared-mime-info,
-spng>=0.7.0, libturbojpeg, LibRaw (optional), librsvg-2.0 (optional),
-xcursor (optional), libtiff (optional), gdk-pixbuf-2.0 (optional)
+spng>=0.7.0, libturbojpeg +
+Optional dependencies: LibRaw, librsvg-2.0, xcursor, libheif, libtiff,
+gdk-pixbuf-2.0
 
  $ git clone --recursive https://git.janouch.name/p/fastiv.git
  $ meson builddir