Fix issue that z is not properly set in some cases when clipping po…

…lygon (#6955)

Fix issue that `z` is not properly set in some cases when clipping the polygon (#6933)

Apply the patch from https://sourceforge.net/p/polyclipping/bugs/160/

src/clipper/clipper.cpp

@@ -2290,7 +2290,11 @@ void Clipper::ProcessHorizontal(TEdge *horzEdge)
 
  if (horzEdge->OutIdx >= 0 && !IsOpen) //note: may be done multiple times
  {
- op1 = AddOutPt(horzEdge, e->Curr);
+#ifdef CLIPPERLIB_USE_XYZ
+ if (dir == dLeftToRight) SetZ(e->Curr, *horzEdge, *e);
+ else SetZ(e->Curr, *e, *horzEdge);
+#endif 
+ op1 = AddOutPt(horzEdge, e->Curr);
  TEdge* eNextHorz = m_SortedEdges;
  while (eNextHorz)
  {
@@ -2614,7 +2618,10 @@ void Clipper::ProcessEdgesAtTopOfScanbeam(const cInt topY)
  {
  e->Curr.x() = TopX( *e, topY );
  e->Curr.y() = topY;
- }
+#ifdef CLIPPERLIB_USE_XYZ
+ e->Curr.z() = topY == e->Top.y() ? e->Top.z() : (topY == e->Bot.y() ? e->Bot.z() : 0);
+#endif
+ }
 
  //When StrictlySimple and 'e' is being touched by another edge, then
  //make sure both edges have a vertex here ...