fix: support top distance was one layer too high (CURA-3269)

src/support.cpp

@@ -342,14 +342,14 @@ void AreaSupport::generateSupportAreas(SliceDataStorage& storage, unsigned int m
     }
 
     //Enforce top Z distance.
-    if (layerZdistanceTop > 0)
+    if (layerZdistanceTop > 1)
     {
         // this is performed after the main support generation loop above, because it affects the joining of polygons
         // if this would be performed in the main loop then some support would not have been generated under the overhangs and consequently no support is generated for that,
         // meaning almost no support would be generated in some cases which definitely need support.
-        for (size_t layer_idx = 0; layer_idx < storage.support.supportLayers.size() && layer_idx < support_layer_count - layerZdistanceTop; layer_idx++)
+        for (size_t layer_idx = 0; layer_idx < storage.support.supportLayers.size() && layer_idx < support_layer_count - (layerZdistanceTop - 1); layer_idx++)
         {
-            supportAreas[layer_idx] = supportAreas[layer_idx].difference(storage.getLayerOutlines(layer_idx + layerZdistanceTop, false));
+            supportAreas[layer_idx] = supportAreas[layer_idx].difference(storage.getLayerOutlines(layer_idx + layerZdistanceTop - 1, false));
         }
     }