Add cvar thresholds to gas overlays (#3835)

* Add cvar thresholds to gas overlays

Rather than having 256 thresholds it can now be reduced to whatever you want

* Address review

* Fix namespace

Content.Shared/CCVar/CCVars.cs

@@ -221,6 +221,9 @@ namespace Content.Shared.CCVar
         public static readonly CVarDef<float> NetGasOverlayTickRate =
             CVarDef.Create("net.gasoverlaytickrate", 3.0f);
 
+        public static readonly CVarDef<int> GasOverlayThresholds =
+            CVarDef.Create("net.gasoverlaythresholds", 20);
+
         /*
          * Admin stuff
          */