Draw outermost line using black relief and erase corners also for cairo.
* xterm.c [USE_CAIRO]: Include math.h. (enum corners) [USE_CAIRO]: New enum. (x_erase_corners_for_relief) [USE_CAIRO]: New function. (x_draw_relief_rect) [USE_CAIRO]: Use it. If box width is larger than 1, draw the outermost line using the black relief.
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