Prevent board-size oscillations

Prevent board-size oscillations

It could happen that after resize there was room for enlarging the
squares by one pixel, but that this crossed the boundary where the line
gaps also become wider, and there is no room for making both squares
and line gap. This would cause perpetual oscillations of the window size.
Resizing is now suppressed when a 1-pixel square-size increase would also
result in a line-gap increase.