Split Untangle's background colour into two. COL_BACKGROUND is still

Split Untangle's background colour into two. COL_BACKGROUND is still
defined the same way and used to draw the normal background of the
puzzle, but there's a new slightly darker COL_SYSBACKGROUND which is
not used for anything at all in the puzzle backend but which occupies
colour slot #0 so that the system will use it to fill around the
playable area if the puzzle is imprecisely resized.

This doesn't _sensibly fix_ the annoyance in Untangle where a non-
square window gives rise to unusable dead space, but it at least works
around it by making the dead space obviously something to stay away
from.

[originally from svn r9823]