move all the Game.Exts junk either down into FileFormats or into WorldUtils

OpenRA.FileFormats/Exts.cs

@@ -105,6 +105,42 @@ namespace OpenRA
 		public static bool Contains(this RectangleF r, int2 p)
 		{
 			return r.Contains(p.ToPointF());
-		}
+		}
+
+        public static bool HasModifier(this Modifiers k, Modifiers mod)
+        {
+            return (k & mod) == mod;
+        }
+
+        public static V GetOrAdd<K, V>(this Dictionary<K, V> d, K k)
+            where V : new()
+        {
+            return d.GetOrAdd(k, _ => new V());
+        }
+
+        public static V GetOrAdd<K, V>(this Dictionary<K, V> d, K k, Func<K, V> createFn)
+        {
+            V ret;
+            if (!d.TryGetValue(k, out ret))
+                d.Add(k, ret = createFn(k));
+            return ret;
+        }
+
+        public static T Random<T>(this IEnumerable<T> ts, Thirdparty.Random r)
+        {
+            var xs = ts.ToArray();
+            return xs[r.Next(xs.Length)];
+        }
+
+        public static float Product(this IEnumerable<float> xs)
+        {
+            return xs.Aggregate(1f, (a, x) => a * x);
+        }
+
+        public static IEnumerable<T> SymmetricDifference<T>(this IEnumerable<T> xs, IEnumerable<T> ys)
+        {
+            // this is probably a shockingly-slow way to do this, but it's concise.
+            return xs.Except(ys).Concat(ys.Except(xs));
+        }
 	}
 }