[!MULTI_FRAME] (syms_of_frame): Make mouse-pixel-position

a synonym for mouse-position, for the benefit of lisp packages
that use it.

src/frame.c

@@ -2463,6 +2463,7 @@ syms_of_frame ()
   defsubr (&Sframe_width);
   Ffset (intern ("screen-width"), intern ("frame-width"));
   defsubr (&Smouse_position);
+  Ffset (intern ("mouse-pixel-position"), intern ("mouse-position"));
   defsubr (&Sframe_parameters);
   defsubr (&Smodify_frame_parameters);
   defsubr (&Sframe_live_p);