Fix infill offset if there are no walls

If there are no walls, the infill area will be equal to the part outline, so it should not be
offsetted.

CURA-7804

Author: konskarm

src/skin.cpp

@@ -416,7 +416,7 @@ void SkinInfillAreaComputation::generateInfill(SliceLayerPart& part, const Polyg
     const size_t wall_line_count = mesh.settings.get<size_t>("wall_line_count");
     const coord_t infill_line_distance = mesh.settings.get<coord_t>("infill_line_distance");
 
-    coord_t offset_from_inner_wall = -innermost_wall_line_width / 2;
+    coord_t offset_from_inner_wall = 0;
     if (wall_line_count > 0)
     { // calculate offset_from_inner_wall
         coord_t extra_perimeter_offset = 0; // to align concentric polygons across layers
@@ -442,7 +442,7 @@ void SkinInfillAreaComputation::generateInfill(SliceLayerPart& part, const Polyg
                 }
             }
         }
-        offset_from_inner_wall += extra_perimeter_offset;
+        offset_from_inner_wall += extra_perimeter_offset - innermost_wall_line_width / 2;
     }
     Polygons infill = part.insets.back().offset(offset_from_inner_wall);
     infill = infill.difference(skin);