Filling: remove directional bias in grid generation.

The method of generating a solved Filling grid (before winnowing
clues) is to loop over every square of the board, and for each one, if
it has a neighbour which is part of a different region of the same
size (i.e. the board is not currently legal), fix it by merging with
one of its neighbours. We pick a neighbour to merge with based on the
size of its region - but we always loop over the four possible
neighbours in the same order, which introduces a directional bias into
the breaking of ties in that comparison.

Now we iterate over the four directions in random order.

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