Fix #73614: gdImageFilledArc() doesn't properly draw pies

Fix #73614: gdImageFilledArc() doesn't properly draw pies

The fix for PHP bug 43828[1] changed the algorithm from drawing filled
pies from drawing multiple triangles to drawing a single polygon.  Due
to quirks of the filled polygon drawing algorithm, we had to filter out
extraneous vertices.  This lead, however, to a bug regarding displaced
starting and ending points near 90° and 270° degrees, which we fix by
reinserting these vertices if they had been removed.

This fix is a port of libgd/libgd@1406b1a.

[1] <https://bugs.php.net/bug.php?id=43828>