fairland: Precompute "exclusive zone" for speed

fairland: Precompute "exclusive zone" for speed

grow_one_sector() and place_island() pass candidate sectors to
try_to_grow() until it succeeds.

try_to_grow() fails when the candidate isn't water, or there are
sectors belonging to other islands within a minimum distance.
It does that the obvious way: it searches for such sectors.

When there is plenty of space, try_to_grow() rarely fails when it
searches.  For each land sector, we visit the sectors within minimum
distance, plus a little extra work for the rare failures.

In a more crowded setup, however, try_to_grow() fails a lot, and we
visit sectors many times more.

Example: Hvy Fever

    8 continents
    continent size: 60
    number of islands: 64
    average size of islands: 10
    spike: 0%
    0% of land is mountain (each continent will have 0 mountains)
    minimum distance between continents: 6
    minimum distance from islands to continents: 3
    World dimensions: 140x68

This is a crowded setup.  With -R 1, try_to_grow() fails almost
750,000 times due to minimum distance, and takes more than 18 Million
iterations.

With default minimum distances 2 and 1, it fails less than 700 times,
taking less than 14,000 iterations.

Instead of searching the surrounding sectors every time we check a
candidate, precompute an "exclusive zone" around each island where
only this island may grow the obvious way: when adding a sector, visit
the sectors within minimum distance and add them to the island's
exclusive zone.

When @extra_distance is non-zero, try_to_grow() still has to search,
Only place_island() passes non-zero @extra_distance.  The next few
commits will get rid of that.

Complication: since the minimum distance for growing islands may
differ from the minimum distance for growing continents, we have to
recompute exclusive zones between growing continents and islands.

For the Hvy Fever example above, this reduces the number of sector
visits by more than 90%, and run time by more than 70% for me.  With
default distances, it's a wash.

Of course, fairland performance is hardly an issue on today's
machines: it takes fairly impractical setups to push run time over a
second.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>