PNG Datatype Handler
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Here is a datatype encoder/decoder based upon libpng and zlib.  It can
read and write the free PNG format images and has a full preference
view.

N.B. When you pass 8bit data to the convert to the PNG format, the
datatype will use the system_colors()->color_list to expand the data
into 24bit.

If there is an PNG colour format this does not support properly I 
would be very grateful if you would send me the offending image.

History
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1.00 - 	Inital Release

Next Version
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Datatype progressive image decoding support.  Save as interlaced
image support.  Different image depth transforms and grayscale.
Quicker saving of images.

Thanks To
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Jon Watte and Jeremy Moskovish

Contact
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Simon Clarke (S.J.Clarke@herts.ac.uk)
If you want to include this datatype with a commerical program, please
email me.