Filling grid gen: slightly randomise neighbour selection.

This is another modification to the same piece of code as the previous
commit. Previously, a square with a neighbour in a same-sized region
was fixed by choosing a neighbour to merge it with that was part of
the smallest region. Now, it's _usually_ that, but sometimes it can be
a larger neighbour instead.

Partly, I hope this might remove a potential source of regularity in
the random grids. But mostly, it prevents the grid generator from
hanging completely on 2x2 grids (e.g. if you gave "2x2#12345" in the
previous state of the code), because with the previous 'always
minimal' rule, the generator would merge together two squares of the
2x2 grid, then the other two, and then (due to maxsize==3) it would
have no merge remaining to clear the final error. Now, every so often,
it will take the unusual option of making a size-3 region instead,
which allows game generation to succeed.

filling.c

@@ -434,7 +434,7 @@ retry:
                 /* find the smallest neighbour to merge with, which
                  * wouldn't make the region too large.  (This is
                  * guaranteed by the initial value of `min'.) */
-                if (neighbour_size < min) {
+                if (neighbour_size < min && random_upto(rs, 10)) {
                     min = neighbour_size;
                     merge = neighbour;
                 }