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.FileFormats/Graphics/IGraphicsDevice.cs

@@ -80,6 +80,8 @@ namespace OpenRA.FileFormats.Graphics
 		void SetData(Bitmap bitmap);
 		void SetData(uint[,] colors);
 		void SetData(byte[] colors, int width, int height);
+		byte[] GetData();
+		Size Size { get; }
 	}
 
 	public interface IFrameBuffer