Portable Network Graphics (PNG) decoder (more or less) for Common Lisp.

All critical chunks are implemented.  Most ancillary chunks  are. It should be able to read back any
file produced by ZPNG. Interlaced files can be read, but without progressive loading.

Usage: read-png-file (file)  and read-png-datastream (stream), both return a  png-state object and a
boolean to  indicate if CRC  was successful,  which most interesting  property is image-data,  a two
(grayscale) or  three dimensional array containing  image data. Other properties  are width, height,
bit-depth and  colour-type. If file was loaded by read-png-file slot png-file contains pathname used.

Dynamic variable *crc-fail-behaviour* can  be set to one of :error, :warn  or :no-action (default is :error),  and   controls  what   happens  on   checksum  failure.   Error   signalled  is   of  type png-read:crc-failure, and restart png-read:ignore-crc-failure is established.

For indexed  images slot index-data  contains an  array with indices  in pallete (stored  in pallete
slot), and image-data contains decoded colors.

The library  loads all files from  http://www.schaik.com/pngsuite/ , but  I did not check  whether it
does so correctly.

Note that ancillary chunks are mostly untested. They never modify primary image data array directly,
but provide additional  information in slots of png-state object. Of  the ancillary chunks specified
in http://www.w3.org/TR/2003/REC-PNG-20031110/  missing are: cHRM,  iCCP and sPLT. gAMA  is present,
but using it is responsibility of library user.

Dynamic variable *warn-missing-ancillaries* controls whether library warns on encountering unknown
ancillary chunk.

Ancillary chunks: 
  tRNS - if present, additional alpha map is stored in slot transparency 
  gAMA - if  present, image gamma is stored in slot gamma 
  sBIT - if present, the original number of significant bits is stored 
         in slot significant-bits as a property list
  sRGB - if present, rendering intent is storen in slot rendering-intent as a keyword symbol
  tEXt - if present, text strings are collected in slot textual-data as an association list of keyword 
         and string
  zTXt - if present, text strings are collected in slot textual-data as an association list of keyword 
         and string
  iTXt - if present, text strings are collected in slot texttual-data as an association list of keyword 
         and a list of language tag, translated keyword and translated text
  bKGD - if present, preferred background is stored in slot preferred-background as an array of color 
         components (RGB), greyscale datum or palette index
  hIST - if present, image histogram is stored in slot image-histogram as an array
  pHYS - if present, intended pixel size is stored in slot physical-dimensions as a property list
  tIME - if present, last modification time is stored in slot last-modification as an universal time