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

src/libslic3r/Layer.hpp

@@ -28,6 +28,10 @@ public:
     // collection of surfaces generated by slicing the original geometry
     // divided by type top/bottom/internal
     SurfaceCollection           slices;
+    // Backed up slices before they are split into top/bottom/internal.
+    // Only backed up for multi-region layers or layers with elephant foot compensation.
+    //FIXME Review whether not to simplify the code by keeping the raw_slices all the time.
+    ExPolygons                  raw_slices;
 
     // collection of extrusion paths/loops filling gaps
     // These fills are generated by the perimeter generator.
@@ -125,8 +129,10 @@ public:
     // Test whether whether there are any slices assigned to this layer.
     bool                    empty() const;    
     void                    make_slices();
-    // Merge typed slices into untyped slices. This method is used to revert the effects of detect_surfaces_type() called for posPrepareInfill.
-    void                    merge_slices();
+    // Backup and restore raw sliced regions if needed.
+    //FIXME Review whether not to simplify the code by keeping the raw_slices all the time.
+    void                    backup_untyped_slices();
+    void                    restore_untyped_slices();
     // Slices merged into islands, to be used by the elephant foot compensation to trim the individual surfaces with the shrunk merged slices.
     ExPolygons              merged(float offset) const;
     template <class T> bool any_internal_region_slice_contains(const T &item) const {