Apparently some layers in PSD files can have extra channels which we
did not account for when computing destination offsets, causing crashes.

So let's make sure we don't include the "extra" channels when computing
the offsets by introducing base_channels. We also move the alpha channel
in the spot of the first extra channel (even though it seems, based on
the one example we have, that the extra channel might be the same as
the alpha channel).