Phil Cazzola:

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.