Flag unrecognised critical tags, but allow ancilliary tags between IDAT and IEND
PNG spec 1.2, section 4.3, turns out tIME iTXt tEXt zTXt have no ordering constraints. Images with these after the IDATs but before IEND were being rejected, plus it's possible to encounter private ancilliary chunks - so it's safest just to let them all through. Unexpected critical chunks are now explicitly faulted. Version 1.46. Tagged as 'ChangeFSI-1_46'
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