Fix for GiST penalty

Hi!

During my work on GSoC project, I found bad perfomace of GiST for point
datatype. It appears even on uniform random data.

test=# create table test as (select point(random(), random()) from
generate_series(1, 1000000));
SELECT 1000000
test=# create index test_idx on test using gist(point);
CREATE INDEX
test=# explain (analyze, buffers) select * from test where point <@
box(point(0.5,0.5), point(0.505,0.505));
QUERY PLAN

------------------------------------------------------------------------------------------------------------------------
Bitmap Heap Scan on test (cost=48.40..2593.73 rows=1000 width=16) (actual
time=97.479..97.551 rows=24 loops=1)
Recheck Cond: (point <@ '(0.505,0.505),(0.5,0.5)'::box)
Buffers: shared hit=5126
-> Bitmap Index Scan on test_idx (cost=0.00..48.15 rows=1000 width=0)
(actual time=97.462..97.462 rows=24 loops=1)
Index Cond: (point <@ '(0.505,0.505),(0.5,0.5)'::box)
Buffers: shared hit=5102
Total runtime: 97.644 ms
(7 rows)

Search for the cause takes relatively long time from me, but finally I did.
In gist_box_penalty function floating point error in line
*result = (float) (size_box(ud) - size_box(origentry->key));
sometimes makes *result a very small negative number.
I beleive that best place to fix it is gistpenalty function. The attached
patch makes this function treating negative number from user's penalty as
zero. I didn't find mention of negative penalty value in documentation. So,
AFAICS such behaviour shouldn't break anything.
After the patch index performance is ok.

test=# explain (analyze, buffers) select * from test where point <@
box(point(0.5,0.5), point(0.505,0.505));
QUERY PLAN

----------------------------------------------------------------------------------------------------------------------
Bitmap Heap Scan on test (cost=44.35..2589.68 rows=1000 width=16) (actual
time=0.988..1.116 rows=24 loops=1)
Recheck Cond: (point <@ '(0.505,0.505),(0.5,0.5)'::box)
Buffers: shared hit=44
-> Bitmap Index Scan on test_idx (cost=0.00..44.10 rows=1000 width=0)
(actual time=0.966..0.966 rows=24 loops=1)
Index Cond: (point <@ '(0.505,0.505),(0.5,0.5)'::box)
Buffers: shared hit=20
Total runtime: 1.313 ms
(7 rows)

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With best regards,
Alexander Korotkov.