Added FSO.Files for use with the API server

Don't ask me. FreeSO is the prime example of dependency hell.

server/tso.files/Utils/WorldTexture.cs

@@ -0,0 +1,13 @@
+using Microsoft.Xna.Framework.Graphics;
+
+namespace FSO.Files.Utils
+{
+    /// <summary>
+    /// A texture used in the game world.
+    /// </summary>
+    public class WorldTexture
+    {
+        public Texture2D Pixel;
+        public Texture2D ZBuffer;
+    }
+}