gst_all_1.gst-plugins-bad: allow building gtksink plugin

Corebird requires gtksink gstreamer plugin to play videos. [1] The plugin,
however, is only built when GTK is available.

This patch adds gtk3 as an optional dependency to gst_all_1.gst-plugins-bad
package, allowing the build of gtksink.

[1]: https://github.com/baedert/corebird/issues/431

pkgs/development/libraries/gstreamer/bad/default.nix

@@ -1,5 +1,6 @@
 { stdenv, fetchurl, pkgconfig, python, gst-plugins-base, orc
 , faacSupport ? false, faac ? null
+, gtkSupport ? false, gtk3 ? null
 , faad2, libass, libkate, libmms
 , libmodplug, mpeg2dec, mpg123
 , openjpeg, libopus, librsvg
@@ -10,6 +11,7 @@
 }:
 
 assert faacSupport -> faac != null;
+assert gtkSupport -> gtk3 != null;
 
 let
   inherit (stdenv.lib) optional optionalString;
@@ -41,7 +43,7 @@ stdenv.mkDerivation rec {
 
   buildInputs = [
     gst-plugins-base orc
-    faad2 libass libkate libmms
+    faad2 gtk3 libass libkate libmms
     libmodplug mpeg2dec mpg123
     openjpeg libopus librsvg
     fluidsynth libvdpau
@@ -50,6 +52,8 @@ stdenv.mkDerivation rec {
   ]
     ++ libintlOrEmpty
     ++ optional faacSupport faac
+    # for gtksink
+    ++ optional gtkSupport gtk3
     ++ optional stdenv.isLinux wayland
     # wildmidi requires apple's OpenAL
     # TODO: package apple's OpenAL, fix wildmidi, include on Darwin