add Filter.NthEntity

src/ComponentDepot.cs

@@ -185,6 +185,11 @@ namespace MoonTools.ECS
 			}
 		}
 
+		public Entity FilterNthEntity(Filter filter, int index)
+		{
+			return new Entity(filterSignatureToEntityIDs[filter.Signature][index]);
+		}
+
 		public Entity FilterRandomEntity(Filter filter)
 		{
 			var randomIndex = RandomGenerator.Next(FilterCount(filter));

src/Filter.cs

@@ -20,5 +20,8 @@ namespace MoonTools.ECS
 
 		public int Count => ComponentDepot.FilterCount(this);
 		public bool Empty => Count == 0;
+
+		// WARNING: this WILL crash if the index is out of range!
+		public Entity NthEntity(int index) => ComponentDepot.FilterNthEntity(this, index);
 	}
 }