Unit cap (#4689)

* Add unit caps as a map setting.

* Use int and add maximum and minimum handling.

* Put a lower limit of 0 for unit cap

core/src/mindustry/entities/Units.java

@@ -76,7 +76,7 @@ public class Units{
         if((team == state.rules.waveTeam && !state.rules.pvp) || (state.isCampaign() && team == state.rules.waveTeam)){
             return Integer.MAX_VALUE;
         }
-        return state.rules.unitCap + indexer.getExtraUnits(team);
+        return Math.max(0, state.rules.unitCapVariable ? state.rules.unitCap + indexer.getExtraUnits(team) : state.rules.unitCap);
     }
 
     /** @return whether this player can interact with a specific tile. if either of these are null, returns true.*/