Fix loading the alpha channel from Photoshop TIFF files

Photoshop apparently thinks that EXTRASAMPLE_UNSPECIFIED specifies
correctly that the extra sample is an alpha channel. This is not
true, but everyone thinks they can mess with the tiff standard, so
just read it like that.

BUG:376950.
(cherry picked from commit dbc50b47f101282690b0db2497773ef32b639de5)