nixos/compton: fix corrupt colours with Mesa 18 on AMD

On AMD hardware with Mesa 18, compton renders some colours incorrectly
when using the glx backend. This patch sets an environmental variable
for compton so colours are rendered correctly.

Topical bug: <https://bugs.freedesktop.org/show_bug.cgi?id=104597>

nixos/modules/services/x11/compton.nix

@@ -238,6 +238,12 @@ in {
       description = "Compton composite manager";
       wantedBy = [ "graphical-session.target" ];
       partOf = [ "graphical-session.target" ];
+
+      # Temporarily fixes corrupt colours with Mesa 18
+      environment = mkIf (cfg.backend == "glx") {
+        allow_rgb10_configs = "false";
+      };
+
       serviceConfig = {
         ExecStart = "${cfg.package}/bin/compton --config ${configFile}";
         RestartSec = 3;