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