PNG allows identical images to be stored differently what makes nearly all
tests checking the MD5 hash of the PNG representation fail with external
libgd. For now, we use the GD format instead, which doesn't allow for such
differences.
Of course, this md5() checking should be replaced by a image diffing feature
in the long run.
No pixel of a filled arc must ever be drawn multiple times. Otherwise we get
artifacts regarding transparency. That happens with the current
implementation of gdImageFilledArc() unless gdChord or gdNoFill are set.
When gdPie is set, however, the filled arc is drawn in wedges, which are
polygons of three points, and so some overlap is natural.
To resolve the issue, we stick with the current algorithm of calculating the
wedges, but instead of drawing each polygon separately, we put the relevant
points in a large array, and draw a single polygon. That also is supposed to
improve the performance considerably.
Note that this modification will change the results when gdImageSetStyle()
or gdImageSetBrush() are used, but we believe that this modification is also
an improvement in this regard, even though it still might not make much
sense to use these functions with gdImageFilledArc().
The respective fix for libgd is
<https://github.com/libgd/libgd/commit/e7e20d6>.
