Path: Fix for optimizeLinearSegments

Reduce cumulative error by holding onto the original point in a sequence
of linear optimizations. In testing, this fixes artifacts I have seen on
very finely sampled paths with very gradual curvature.

Further improvements are needed for this optimization, but at least this
change avoids a regression over the status quo.

src/Mod/Path/PathScripts/PathSurface.py

@@ -2093,7 +2093,7 @@ def _optimizeLinearSegments(self, line):
             for nxt in line[2:]:
                 if not pnt.isOnLineSegment(prv, nxt):
                     pts.append(pnt)
-                prv = pnt
+                    prv = pnt
                 pnt = nxt
             pts.append(line[-1])
             return pts