Fix of fan control for raft layers.

Fixes Fan starts at first layer, even though disabled for first layer. #7232
This is a regression due to cooling refactoring, which cooled
support layers independently from object layers. The bug here was that
all the raft layers were cooled together with the first object layer.

src/libslic3r/SupportMaterial.cpp

@@ -1480,7 +1480,7 @@ static inline std::tuple<Polygons, Polygons, Polygons, float> detect_overhangs(
         overhang_polygons = to_polygons(layer.lslices);
 #endif
         // Expand for better stability.
-        contact_polygons = expand(overhang_polygons, scaled<float>(object_config.raft_expansion.value));
+        contact_polygons = object_config.raft_expansion.value > 0 ? expand(overhang_polygons, scaled<float>(object_config.raft_expansion.value)) : overhang_polygons;
     }
     else if (! layer.regions().empty())
     {