Expose FBOs to engine code.

OpenRA.Renderer.Null/NullGraphicsDevice.cs

@@ -59,6 +59,7 @@ namespace OpenRA.Renderer.Null
 		public IVertexBuffer<Vertex> CreateVertexBuffer(int size) { return new NullVertexBuffer<Vertex>(); }
 		public ITexture CreateTexture() { return new NullTexture(); }
 		public ITexture CreateTexture(Bitmap bitmap) { return new NullTexture(); }
+		public IFrameBuffer CreateFrameBuffer(Size s) { return new NullFrameBuffer(); }
 		public IShader CreateShader(string name) { return new NullShader(); }
 	}
 
@@ -80,6 +81,13 @@ namespace OpenRA.Renderer.Null
 		public void SetData(byte[] colors, int width, int height) { }
 	}
 
+	public class NullFrameBuffer : IFrameBuffer
+	{
+		public void Bind() { }
+		public void Unbind() { }
+		public ITexture Texture { get { return new NullTexture(); } }
+	}
+
 	class NullVertexBuffer<T> : IVertexBuffer<T>
 	{
 		public void Bind() { }