tRNS method sets (TRANSPARENCY *PNG-STATE*) to some raw data
representation depending on colour-type. Then it adds
so (TRANSPARENCY *PNG-STATE*) will become a 2D-array
before *png-state* is finished.
The latter part (adding the postprocessor) was inside an incorrect
WHEN form, requiring (COLOUR-TYPE *PNG-STATE*) to be 0 or 2. First,
our COLOUR-TYPE is a keyword, not a byte (thus 0 and 2 correspond to
:greyscale and :truecolor, respectively). Second,
build-transparency-map is prepared to handle :indexed-colour images as
well, and there are images where it's useful. Third, preceding ECASE
will fail for any colour-type except :greyscale, :truecolor and
Therefore it seems that no additional checks are needed here at all,
and that's the assumption behind this patch.