make it easier to discern where is up and where is down.
- solidify/gui_util.h, solidify/gui_util.c (hpoint, vpoint, alpha, aa_line):
draw horizontal or vertical lines with anti-aliases beginning and end
- solidify/level.c (draw_xz, draw_zy): instead of drawing thin lines for z
and z0, show a filled surface of the difference
- solidify/util.h, solidify/util.c (draw_circle), solidify/gui_util.h,
solidify/gui_util.c: moved draw_circle from util.c to gui_util.c
- solidify/util.c: this file is now empty. Remove it.
- solidify/level.c, solidify/overlap.c: include gui_util.h instead of
util.h
- solidify/Makefile: replace util.o with gui_util.o
