flatpak: Preserve pixbuf path in validate-icon

SVG support isn't built-in to GdkPixBuf, it's provided by an external
module. Therefore, GDK_PIXBUF_MODULE_FILE should be preserved in the
sandbox, otherwise `flatpak-validate-icon --sandbox` won't work on SVG
files.

pkgs/development/libraries/flatpak/default.nix

@@ -33,6 +33,7 @@ stdenv.mkDerivation rec {
     ./respect-xml-catalog-files-var.patch
     ./use-flatpak-from-path.patch
     ./unset-env-vars.patch
+    ./validate-icon-pixbuf.patch
   ];
 
   nativeBuildInputs = [

pkgs/development/libraries/flatpak/validate-icon-pixbuf.patch

@@ -0,0 +1,13 @@
+diff --git a/icon-validator/validate-icon.c b/icon-validator/validate-icon.c
+index 6e23d9f2..f0659a78 100644
+--- a/icon-validator/validate-icon.c
++++ b/icon-validator/validate-icon.c
+@@ -193,6 +193,8 @@ rerun_in_sandbox (const char *arg_width,
+     add_args (args, "--setenv", "G_MESSAGES_DEBUG", g_getenv ("G_MESSAGES_DEBUG"), NULL);
+   if (g_getenv ("G_MESSAGES_PREFIXED"))
+     add_args (args, "--setenv", "G_MESSAGES_PREFIXED", g_getenv ("G_MESSAGES_PREFIXED"), NULL);
++  if (g_getenv ("GDK_PIXBUF_MODULE_FILE"))
++    add_args (args, "--setenv", "GDK_PIXBUF_MODULE_FILE", g_getenv ("GDK_PIXBUF_MODULE_FILE"), NULL);
+ 
+   add_args (args, validate_icon, arg_width, arg_height, filename, NULL);
+   g_ptr_array_add (args, NULL);