Make custom Env flag reliably possible (#8019)

* Make custom Env flag reliably possible

* screw you

* "compatibility" for mods that don't use add()

core/src/mindustry/io/JsonIO.java

@@ -2,6 +2,7 @@ package mindustry.io;
 
 import arc.graphics.*;
 import arc.math.geom.*;
+import arc.struct.*;
 import arc.util.*;
 import arc.util.serialization.*;
 import arc.util.serialization.Json.*;
@@ -78,6 +79,30 @@ public class JsonIO{
         json.setElementType(Rules.class, "spawns", SpawnGroup.class);
         json.setElementType(Rules.class, "loadout", ItemStack.class);
 
+        json.setSerializer(Rules.class, new Serializer<>(){
+            @Override
+            public void write(Json json, Rules object, Class knownType){
+                json.writeObjectStart();
+                json.writeFields(object);
+                json.writeValue("envMapping", Env.idToName, IntMap.class);
+                json.writeObjectEnd();
+            }
+
+            @Override
+            public Rules read(Json json, JsonValue jsonData, Class type){
+                Rules out = new Rules();
+                json.readFields(out, jsonData);
+
+                // Older Rules data doesn't have env mapping.
+                if(jsonData.has("envMapping")){
+                    IntMap<String> idToName = json.readValue(IntMap.class, jsonData.get("envMapping"));
+                    out.env = Env.remap(out.env, idToName);
+                }
+
+                return out;
+            }
+        });
+
         json.setSerializer(Color.class, new Serializer<>(){
             @Override
             public void write(Json json, Color object, Class knownType){