Box graphic tweak

[Chong Yidong]

I just made a tweak to the way 3D boxes are drawn, to make them more
legible.  The W32 code looks like it could use a similar change, but I
didn't want to do the translation without being able to test it.  Could
someone on W32 take a look?  Thanks.

  * xterm.c (x_draw_relief_rect): If box width is larger than 1,
  draw the outermost line using the black relief, for legibility.
  Omit drawing the four corner pixels.