This is a minor bug fix for sgBucketDiff().
If you crossed the bucket size boundary, the answer for dx could be wrong.
E.g.
going from 0:0, 21:7 to 0:7, 21:7 would give you dx = 7 (correct)
but going from 0:0, 21:7 to 0:3, 22:0 would give you dx = 6 (instead of 7)
Previously it differenced the center longitudes of the buckets. When you
cross a boundary, the center point of the larger bucket now lies on the edge of the smaller bucket.
The result was a dx with an integer + 1/2 bucket, which rint() was rounding to the nearest even int.
This function only seems to be used in TerraGear.
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