[dev] PNG optimization for IE

[Jan Schneider]

A lot of PNGs can be optimized to a) avoid the flicker when loading the
images on slow machines or over slow lines for the first time, and b) don't
look too bad on machines that don't have the AlphaImageLoader installed.

To see what I'm talking about, set the new Horde preference to turn the
alpha loader off and look at the images. Some look normal, some have a
white background, some have grey backrounds, and some a black one.

The images with a grey background are icons with true alpha transparency,
and we can't change anything about that. The icons with white background
are probably stored with alpha transparency even though not all have to.
For icons with less than 256 colors, indexed transparency is enough, and
they would look exactly like transparent GIFs.
I'm not sure about the icons with the black background, but I guess these
had somehow a black background set when creating the transparency, it
should be possible to convert them to at least a white background if not to
indexed transparency and a transparent background.

So much for the theory, correct me anyone where I'm wrong. Volunteers who
know their tools are very much appreciated. Most Horde developers don't use
IE anymore so this is rather a low priority task for us.

Jan.

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