space-frisk now shows edges

[Thien-Thi Nguyen]

new keybinding `e' toggles edge visibility.  midpoint to upstream in
white, downstream to midpoint in a random color.  tarball in dir:

  http://www.glug.org/people/ttn/software/guile-sdl/demos/

the effect shows the limitation of 2.5D (as opposed to "true" 3D); this
will hopefully motivate Guile-SDL to move towards opengl support more
sooner than later.  on the other hand, the limitation can be viewed as
opportunity for a SMOP -- patches welcome.

thi