Fixed MMU segmentation for multi-volume objects.

MMU segmentation no longer works directly on lslices, instead of it works on custom merged regions. So lslices in PrintObject are no longer overwritten because of MMU segmentation.
All regions are scaled by SCALED_EPSILON before merging and shrunk back by SCALED_EPSILON after merging. That fixed issues with multi-volume objects when very close regions weren't merged.
Also, small expolygons and holes are filtered out that fixed missing segmentation at the boundary of two volumes in the case of multi-volume objects.

src/libslic3r/ExPolygon.hpp

@@ -360,6 +360,9 @@ extern std::vector<BoundingBox> get_extents_vector(const ExPolygons &polygons);
 extern bool        remove_sticks(ExPolygon &poly);
 extern void 	   keep_largest_contour_only(ExPolygons &polygons);
 
+// Removes all expolygons smaller than min_area and also removes all holes smaller than min_area
+extern bool        remove_small_and_small_holes(ExPolygons &expolygons, double min_area);
+
 inline double area(const ExPolygons &polys)
 {
     double s = 0.;