This patch makes gdk-pixbuf look for additional modules in a list of files
specified by the environment variable "GUIX_GDK_PIXBUF_MODULE_FILES".

A similiar patch for "GDK_PIXBUF_MODULE_FILES" had been sent to upstream:
  https://gitlab.gnome.org/GNOME/gdk-pixbuf/-/merge_requests/180

We use a "GUIX_" prefixed one to avoid breaking foreign programs:
  https://issues.guix.gnu.org/63853
  https://issues.guix.gnu.org/75523

Upstream-status: N/A
---
diff --git a/gdk-pixbuf/gdk-pixbuf-io.c b/gdk-pixbuf/gdk-pixbuf-io.c
index e1df590..913ce89 100644
--- a/gdk-pixbuf/gdk-pixbuf-io.c
+++ b/gdk-pixbuf/gdk-pixbuf-io.c
@@ -707,6 +707,16 @@ gdk_pixbuf_io_init (void)
 
 	gdk_pixbuf_io_init_builtin ();
 
+        /* Load modules from GUIX_GDK_PIXBUF_MODULE_FILES. */
+        gchar *module_files_env = g_getenv ("GUIX_GDK_PIXBUF_MODULE_FILES");
+        if (module_files_env) {
+            gchar **module_files = g_strsplit (module_files_env,
+                                               G_SEARCHPATH_SEPARATOR_S, 0);
+            for (int i = 0; module_files[i] != NULL; i++)
+                gdk_pixbuf_io_init_modules (module_files[i], NULL);
+            g_strfreev (module_files);
+        }
+
 	return file_formats != NULL;
 }
