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.cpp

@@ -372,6 +372,27 @@ bool remove_sticks(ExPolygon &poly)
     return remove_sticks(poly.contour) || remove_sticks(poly.holes);
 }
 
+bool remove_small_and_small_holes(ExPolygons &expolygons, double min_area)
+{
+    bool   modified = false;
+    size_t free_idx = 0;
+    for (size_t expoly_idx = 0; expoly_idx < expolygons.size(); ++expoly_idx) {
+        if (std::abs(expolygons[expoly_idx].area()) >= min_area) {
+            // Expolygon is big enough, so also check all its holes
+            modified |= remove_small(expolygons[expoly_idx].holes, min_area);
+            if (free_idx < expoly_idx) {
+                std::swap(expolygons[expoly_idx].contour, expolygons[free_idx].contour);
+                std::swap(expolygons[expoly_idx].holes, expolygons[free_idx].holes);
+            }
+            ++free_idx;
+        } else
+            modified = true;
+    }
+    if (free_idx < expolygons.size())
+        expolygons.erase(expolygons.begin() + free_idx, expolygons.end());
+    return modified;
+}
+
 void keep_largest_contour_only(ExPolygons &polygons)
 {
 	if (polygons.size() > 1) {

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.;

src/libslic3r/MultiMaterialSegmentation.cpp

@@ -1357,13 +1357,27 @@ std::vector<std::vector<std::pair<ExPolygon, size_t>>> multi_material_segmentati
     std::vector<std::vector<PaintedLine>>                  painted_lines(print_object.layers().size());
     std::vector<EdgeGrid::Grid>                            edge_grids(print_object.layers().size());
     const ConstLayerPtrsAdaptor                            layers = print_object.layers();
+    std::vector<Polygons>                                  input_polygons(layers.size());
+
+    // Merge all regions and remove small holes
+    for(size_t layer_idx = 0; layer_idx < layers.size(); layer_idx += 1) {
+        ExPolygons ex_polygons;
+        for (LayerRegion *region : layers[layer_idx]->regions())
+            for (const Surface &surface : region->slices.surfaces)
+                Slic3r::append(ex_polygons, offset_ex(surface.expolygon, SCALED_EPSILON));
+        // All expolygons are expanded by SCALED_EPSILON, merged, and then shrunk again by SCALED_EPSILON
+        // to ensure that very close polygons will be merged.
+        ex_polygons = union_ex(ex_polygons);
+        // Remove all expolygons and holes with an area less than 0.01mm^2
+        remove_small_and_small_holes(ex_polygons, Slic3r::sqr(scale_(0.1f)));
+        input_polygons[layer_idx] = to_polygons(union_ex(offset_ex(ex_polygons, -SCALED_EPSILON)));
+    }
 
     for (size_t layer_idx = 0; layer_idx < layers.size(); ++layer_idx) {
-        const Layer *layer = layers[layer_idx];
-        BoundingBox  bbox(get_extents(layer->lslices));
+        BoundingBox  bbox(get_extents(input_polygons[layer_idx]));
         bbox.offset(SCALED_EPSILON);
         edge_grids[layer_idx].set_bbox(bbox);
-        edge_grids[layer_idx].create(layer->lslices, coord_t(scale_(10.)));
+        edge_grids[layer_idx].create(input_polygons[layer_idx], coord_t(scale_(10.)));
     }
 
     for (const ModelVolume *mv : print_object.model_object()->volumes) {
@@ -1453,11 +1467,7 @@ std::vector<std::vector<std::pair<ExPolygon, size_t>>> multi_material_segmentati
             std::vector<PaintedLine> &painted_lines_single = painted_lines[layer_idx];
 
             if (!painted_lines_single.empty()) {
-                Polygons original_polygons;
-                for (const Slic3r::EdgeGrid::Contour &contour : edge_grids[layer_idx].contours())
-                    original_polygons.emplace_back(Points(contour.begin(), contour.end()));
-
-                std::vector<std::vector<ColoredLine>> color_poly = colorize_polygons(original_polygons, painted_lines_single);
+                std::vector<std::vector<ColoredLine>> color_poly = colorize_polygons(input_polygons[layer_idx], painted_lines_single);
                 MMU_Graph                             graph      = build_graph(layer_idx, color_poly);
                 remove_multiple_edges_in_vertices(graph, color_poly);
                 graph.remove_nodes_with_one_arc();

src/libslic3r/PrintObject.cpp

@@ -1906,15 +1906,6 @@ void PrintObject::_slice(const std::vector<coordf_t> &layer_height_profile)
 
     // --------------------MMU_SEGMENTATION_BEGIN----------------------
 
-    // Temporary fix for not assigned lslices
-    for(size_t layer_idx = 0; layer_idx < m_layers.size(); layer_idx += 1) {
-        ExPolygons ex_polygons;
-        for (LayerRegion *region : this->m_layers[layer_idx]->regions())
-            for (const Surface &surface : region->slices.surfaces)
-                ex_polygons.emplace_back(surface.expolygon);
-        this->m_layers[layer_idx]->lslices = union_ex(ex_polygons);
-    }
-
     size_t region_count_before_change = this->num_regions();
     std::vector<std::vector<std::pair<ExPolygon, size_t>>> segmented_regions = multi_material_segmentation_by_painting(*this);
     // Skip region with default extruder