New semantics for ExtrusionLoop objects. Early processing of perimeter overhangs for paralellizing such work and making G-code export lighter. Lots of refactoring. This should fix a number of minor bugs, including reversals of perimeter overhangs.

xs/xsp/Clipper.xsp

@@ -101,6 +101,15 @@ diff_pl(subject, clip)
     OUTPUT:
         RETVAL
 
+Polylines
+diff_ppl(subject, clip)
+    Polygons    subject
+    Polygons    clip
+    CODE:
+        diff(subject, clip, RETVAL);
+    OUTPUT:
+        RETVAL
+
 Polygons
 intersection(subject, clip, safety_offset = false)
     Polygons                    subject
@@ -130,6 +139,15 @@ intersection_pl(subject, clip)
     OUTPUT:
         RETVAL
 
+Polylines
+intersection_ppl(subject, clip)
+    Polygons                    subject
+    Polygons                    clip
+    CODE:
+        intersection(subject, clip, RETVAL);
+    OUTPUT:
+        RETVAL
+
 ExPolygons
 xor_ex(subject, clip, safety_offset = false)
     Polygons                    subject