libva: build a lighter version by default

Mesa, X etc. are not very light dependencies.
This will also allow us to break the cycle with mesa.

pkgs/development/libraries/gstreamer/default.nix

@@ -1,4 +1,4 @@
-{ callPackage }:
+{ callPackage, libva-full }:
 
 rec {
   gstreamer = callPackage ./core { };
@@ -17,5 +17,8 @@ rec {
 
   gst-editing-services = callPackage ./ges { inherit gnonlin; };
 
-  gst-vaapi = callPackage ./vaapi { inherit gst-plugins-base gstreamer gst-plugins-bad; };
+  gst-vaapi = callPackage ./vaapi {
+    inherit gst-plugins-base gstreamer gst-plugins-bad;
+    libva = libva-full; # looks also for libva-{x11,wayland}
+  };
 }