Add stub for SurfaceBox

surfacebox.go

@@ -0,0 +1,8 @@
+package x
+
+import "errors"
+import "git.tebibyte.media/tomo/tomo"
+
+func (backend *Backend) NewSurfaceBox() (tomo.SurfaceBox, error) {
+	return nil, errors.New("SurfaceBox not implemented yet")
+}