Support PLAs with alternating inverted rows.

[peterbjornx]

The program currently only supports PLAs laid out as nA A nB B nC C ... nZ Z, or A nA B nB C nC ... Z nZ.

Some PLAs instead use nA A B nB nC C ... , or A nA nB B C nC ....

My projects currently handle this in the post processing but this should be part of the decoder software