[boost] [GIL]: extended png io

From: Andreas Pokorny (andreas.pokorny_at_[hidden])
Date: 2007-12-23 03:14:28

Hello Lubomir, Hailin and the other boosters,
I implemented support for more png file types, including
palette and grayscale with alpha images.
For the implementation I had to add a gray_alpha layout,
and extend the color_type metafunction for homogeneous
bit_aligned_pixel_* types - packed_pixel is still missing.

I tested 1,2 and 4 bit images with
http://www.schaik.com/pngsuite/pngsuite_bas_png.html
and some images with

I also added additional calls to png_packswap to get proper
results for 1bit images.

Could you review the changes?

A svn diff of boost/gil/ is attached to the email. I have a boost
svn account, and could add that stuff myself, if you want me to.
I could also add more test cases that capture more of the
png file types.

kind regards
Andreas Pokorny