Pattern: randomise rounding bias in generate().

Pattern: randomise rounding bias in generate().

Now, with an odd grid size, we choose the posterisation threshold so
that half the time it delivers ceil(n/2) black squares and half the
time it delivers floor(n/2). Previously it only did the former, which
meant that asking Pattern to generate a 1x1 puzzle (with the bug in
the previous commit fixed) would always generate the one with a single
black square, and never the one with a single white square. Both are
trivial to solve, of course, but it seemed inelegant!

No change to the number of black squares in the puzzle solution can
constitute a spoiler for the player, of course, because that number is
trivial to determine without doing any difficult reasoning, just by
adding up all the clues in one dimension.