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
- solidify/level.c (expose_event, level): renamed expose_event to
expose_event_xy, for consistency with new expose event handlers
- solidify/level.c (expose_event_xz, expose_event_zy, level): added expose
event handlers for the auxiliary projections
- solidify/level.c (level): the drawing areas receive an expose event when
becoming visible, so there's no need to explicitly initilize them
- solidify/level.c (xz, draw_xz, motion_notify_event, level): show a
projection on a xz plane at the cursor position
- solidify/level.c (zy, draw_zy, motion_notify_event, level): show a
projection on a zy plane at the cursor position
