pstext -D[j|J] is broken

[Chuan1937]

For MC (Middle Center) alignment, the just value is 6. When calculating the offset in gmt_smart_justify:

xx = (2 - (just % 4)) * dx = 0
yy = (1 - (just / 4)) * dy = 0

So when text is centered horizontally or vertically, the offset in the corresponding direction becomes 0, rendering the -Dj option ineffective.

Solution: When text is centered in a given direction, use the default offset direction (+1) instead of 0:

Fixes #8712

[joa-quim]

Thanks for looking into this.