If user chooses to export palette image type, pngcrush
should automatically truncate unused colours in the
palette, and reduces bit depth where appropriate,
without using -plte_len switch.
I wrote the PNG palette-reduction for ImageMagick. It was a huge undertaking that I don't want to duplicate in pngcrush. I recommend doing a first pass with ImageMagick, then using pngcrush on the result.
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