Fix board addressing in custom layout editor

[mrcity]

The board addresses calculated by the panels electrically during initialization do not match the addresses calculated by the custom layout editor. This seems to be related to some kind of difference in rounding the calculated address, and does not get sufficiently compensated for by the current correction scheme.

On a somewhat related matter, the correction scheme in software does not account for when you use the terminator on the left rather than on the right. This tends to cause the board addresses to go higher than expected, rather than to "droop" lower than expected in the middle of the marquee.