Fix of re-slicing with multiple regions.

This is a fix of a bug, which was in Slic3r forever, where raw slices
were not cached, but recalculated from classified regions, where
merging the regions did not produce the original contour reliably.
Fixes [2.3.0-beta2] Odd bad slicing related to infill (?) percentage #5407

xs/xsp/Layer.xsp

@@ -70,7 +70,8 @@
         %code%{ RETVAL = dynamic_cast<SupportLayer*>(THIS); %};
     
     void make_slices();
-    void merge_slices();
+    void backup_untyped_slices();
+    void restore_untyped_slices();
     void make_perimeters();
     void make_fills();