libjpeg/libpng-based image loading

[Alexander Malmberg]

Hi,

Stefan Kleine Stegemann write a new libjpeg-based jpeg loader, I wrote a
libpng-based png loader, and Adam wrote type 5/6 pnm loader. I've
combined these parts into the following patch:

http://w1.423.telia.com/~u42308495/alex/image_loading_patch.tar.gz

This also disables the current libwraster loading hacks, and my plan is
to remove these entirely once savannah is back up and I can commit
again. Until then, testing and feedback is appreciated.

When this is done, we'll support (loading) the following formats:
* tiff
* jpeg
* png
* pnm (type 5, aka pgm, and type 6, aka ppm)

If your favorite format isn't in this list, now would be a good time to
write a loader for it. :)

(Any of the new loader categories would be a good starting point. If we
end up with many additional loaders, I'll probably rearrange the
NSBitmapImageRep parts a bit, but that won't affect the loader
categories much.)

- Alexander Malmberg