Logic simplification in `lepton_encode_row_range/decode_row_wrapper`

[Melirius]

As mid_right is indistinguishable from middle and width_one from mid_left, logic inside these functions can be simplified, as well as lower in process_row and decode_row.

[mcroomp]

I think its vestiges of using the upper right block which never got implemented as far as I can tell, but yeah it can be dramatically simplified. Even one_width isn't necessary.

I had this change that I was working on #82