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Really invoke build-transparency-map when a tRNS chunk is present.
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 :indexed-colour. Therefore it seems that no additional checks are needed here at all, and that's the assumption behind this patch.
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