Texture changes:

* The GL texture id is now readonly.
* Added Size property.
* Added GetData() for reading data back from the GPU.
* Added SetEmpty() for creating an empty texture of a given size.

OpenRA.Renderer.Null/NullGraphicsDevice.cs

@@ -79,6 +79,8 @@ namespace OpenRA.Renderer.Null
 		public void SetData(Bitmap bitmap) { }
 		public void SetData(uint[,] colors) { }
 		public void SetData(byte[] colors, int width, int height) { }
+		public byte[] GetData() { return new byte[0]; }
+		public Size Size { get { return new Size(0, 0); } }
 	}
 
 	public class NullFrameBuffer : IFrameBuffer