Adding vaapi drivers to nixos

Enabling nvidia and intel drivers by default. I tried both,
and they work fine with "vlc --ffmpeg-hw".

modules/services/x11/display-managers/default.nix

@@ -16,6 +16,17 @@ let
   cfg = config.services.xserver;
   xorg = pkgs.xorg;
 
+  vaapiDrivers = pkgs.buildEnv {
+    name = "vaapi-drivers";
+    paths = cfg.vaapiDrivers;
+    pathsToLink = "/lib/dri";
+
+    # To admit zero-length 'paths'
+    postBuild = "
+      ensureDir $out/lib/dri
+    ";
+  };
+
   # file provided by services.xserver.displayManager.session.script
   xsession = wm: dm: pkgs.writeScript "xsession"
     ''
@@ -80,6 +91,8 @@ let
           ${xorg.xrdb}/bin/xrdb -merge ~/.Xdefaults
       fi
 
+      export LIBVA_DRIVERS_PATH=${vaapiDrivers}/lib/dri;
+
       source /etc/profile
 
       ${cfg.displayManager.sessionCommands}

modules/services/x11/xserver.nix

@@ -30,7 +30,6 @@ let
     (name: { inherit name; driverName = name; } //
       attrByPath [name] (if (hasAttr ("xf86video" + name) xorg) then { modules = [(getAttr ("xf86video" + name) xorg) ]; } else throw "unknown video driver `${name}'") knownVideoDrivers);
 
-
   fontsForXServer =
     config.fonts.fonts ++
     # We don't want these fonts in fonts.conf, because then modern,
@@ -204,6 +203,15 @@ in
         '';
       };
 
+      vaapiDrivers = mkOption {
+        default = [ pkgs.vaapiIntel pkgs.vaapiVdpau ];
+        defaultText = "[ pkgs.vaapiIntel pkgs.vaapiVdpau ]";
+        example = "[ pkgs.vaapiIntel pkgs.vaapiVdpau ]";
+        description = ''
+          Names of the packages providing libva acceleration drivers.
+        '';
+      };
+
       driSupport = mkOption {
         default = true;
         description = ''