gnu: libpng-apng: Update to 1.6.39.

I've tweaked the comment, as it made me read an entire e-mail thread
only to realise I needn't have done so.

* gnu/packages/image.scm (libpng-apng): Update to 1.6.39.

gnu/packages/image.scm

@@ -178,12 +178,11 @@ library.  It supports almost all PNG features and is extensible.")
    (license license:zlib)
    (home-page "http://www.libpng.org/pub/png/libpng.html")))
 
-;; libpng-apng should be updated when the APNG patch is released:
-;; <https://bugs.gnu.org/27556>
 (define-public libpng-apng
+  ;; The APNG patch is maintained separately and may lag behind upstream libpng.
   (package
     (name "libpng-apng")
-    (version "1.6.37")
+    (version "1.6.39")
     (source
      (origin
        (method url-fetch)
@@ -196,8 +195,7 @@ library.  It supports almost all PNG features and is extensible.")
                    "ftp://ftp.simplesystems.org/pub/libpng/png/src/history"
                    "/libpng16/libpng-" version ".tar.xz")))
        (sha256
-        (base32
-         "1jl8in381z0128vgxnvn33nln6hzckl7l7j9nqvkaf1m9n1p0pjh"))))
+        (base32 "0dv90dxvmqpk7mbywyjbz8lh08cv4b0ksqp1y62mzvmlf379cihz"))))
     (build-system gnu-build-system)
     (arguments
      `(#:modules ((guix build gnu-build-system)
@@ -231,7 +229,7 @@ library.  It supports almost all PNG features and is extensible.")
                                   version "/libpng-" version "-apng.patch.gz"))
                   (sha256
                    (base32
-                    "1dh0250mw9b2hx7cdmnb2blk7ddl49n6vx8zz7jdmiwxy38v4fw2"))))))
+                    "1z8cx011a2c7vagwgi92rbmky1wi8awmrdldqh9f5k80pbmbdi2a"))))))
     (native-inputs
      (list libtool))
     ;; libpng.la says "-lz", so propagate it.