Implement IEquatable on structs.

Any struct which overrides object.Equals(object obj) should implement IEquatable<T> to gain a more efficient Equals(T other) overload. This overload will be used by hashing collections like Dictionary which enables them to check equality without boxing the struct.

OpenRA.Game/Primitives/Size.cs

@@ -13,7 +13,7 @@ using System;
 
 namespace OpenRA.Primitives
 {
-	public struct Size
+	public struct Size : IEquatable<Size>
 	{
 		public readonly int Width;
 		public readonly int Height;
@@ -46,6 +46,11 @@ namespace OpenRA.Primitives
 
 		public bool IsEmpty { get { return Width == 0 && Height == 0; } }
 
+		public bool Equals(Size other)
+		{
+			return this == other;
+		}
+
 		public override bool Equals(object obj)
 		{
 			if (!(obj is Size))