Implement error checking in Slant. Clue points are now highlighted

Implement error checking in Slant. Clue points are now highlighted
in red if it's impossible to fulfill them (either through too many
neighbours connecting to them, or too many not connecting to them),
and edges are highlighted in red if they form part of a loop.

In order to do this I've had to revamp the redraw function
considerably. Each square is now drawn including its top and left
grid edges, but _not_ its bottom or right ones - which means that I
need to draw an extra strip of empty squares outside the actual grid
in order to draw the few pixels which appear on the grid bottom and
right borders and also to red-highlight border clues.

[originally from svn r6174]