expo: Provide a way to position things relative to display

It is often necessary to centre objects within the display area. Add a
special position value to indicate this.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/expo.h

@@ -242,6 +242,12 @@ struct scene_obj_dims {
 	int y;
 };
 
+/* special values for dimensions */
+enum {
+	/* width/height of the display */
+	SCENEOB_DISPLAY_MAX	= 0x7f000000,
+};
+
 /**
  * enum scene_obj_halign - Horizontal alignment of objects
  *