Tile the booleans on layers of many pieces

ClipperLib slows down with the number of edges on a scan line, and a
layer cut through a fine relief has tens of thousands of pieces.
detect_surfaces_type ~50 s at 0.1 mm / 2000k, was ~145.

src/libslic3r/LayerRegion.cpp

@@ -72,10 +72,11 @@ void LayerRegion::slices_to_fill_surfaces_clipped()
         by_surface[size_t(surface.surface_type)].emplace_back(&surface);
     // Trim surfaces by the fill_boundaries.
     this->fill_surfaces.surfaces.clear();
+    const Polygons fill_boundaries = to_polygons(this->fill_expolygons);
     for (size_t surface_type = 0; surface_type < size_t(stCount); ++ surface_type) {
         const SurfacesPtr &this_surfaces = by_surface[surface_type];
         if (! this_surfaces.empty())
-            this->fill_surfaces.append(intersection_ex(this_surfaces, this->fill_expolygons), SurfaceType(surface_type));
+            this->fill_surfaces.append(intersection_ex_by_piece(to_expolygons(this_surfaces), fill_boundaries), SurfaceType(surface_type));
     }
 }