Add readonly to structs

OpenRA.Game/Graphics/PlatformInterfaces.cs

@@ -145,7 +145,7 @@ namespace OpenRA
 		TriangleList,
 	}
 
-	public struct Range<T>
+	public readonly struct Range<T>
 	{
 		public readonly T Start, End;
 		public Range(T start, T end) { Start = start; End = end; }