Fix #64641: imagefilledpolygon doesn't draw horizontal line

Fix #64641: imagefilledpolygon doesn't draw horizontal line

As has been reported, 1-dimensional horizontal filled polygons are not drawn
at all. That is caused by the scanline algorithm used for drawing filled
polygons which skips the drawing of horizontal edges, because that is
normally not necessary. If, however, the polygon consists of horizontal
edges only, that obviously doesn't work, so we add a special case handling.

That has also been fixed in libgd with
<https://github.com/libgd/libgd/commit/f9f10fa9>.