The alpha values in parcellations created by FreeSurfer are always zero, which would usually mean that the labels are transparent, but in FS it means the opposite. Previously, `_read_parc` just ignored this value and always returned 255 for it. This just makes sure that an alpha value of 0 is mapped to 255, and 255 is mapped to 0. This is also done in the writing function, so it is still compatible with nibabel etc.