xserver service: add glamoregl for intel drivers

Closes #14286

Credits to vcunat for the initial patch.

nixos/modules/services/x11/xserver.nix

···
 
   # Map video driver names to driver packages. FIXME: move into card-specific modules.
   knownVideoDrivers = {
-    virtualbox   = { modules = [ kernelPackages.virtualboxGuestAdditions ]; driverName = "vboxvideo"; };
-    ati = { modules = [ pkgs.xorg.xf86videoati pkgs.xorg.glamoregl ]; };
-    intel-testing = { modules = with pkgs.xorg; [ xf86videointel-testing glamoregl ]; driverName = "intel"; };
+    virtualbox = { modules = [ kernelPackages.virtualboxGuestAdditions ]; driverName = "vboxvideo"; };
+    ati = { modules = with pkgs.xorg; [ xf86videoati glamoregl ]; };
+    intel = { modules = with pkgs.xorg; [ xf86videointel glamoregl ]; };
   };
 
   fontsForXServer =