FIX: FillBed crashes when accessing unset print config option

1. When the option is not set, print.full_print_config() is empty, so we
can't access opt_bool("scan_first_layer") or opt_bool("enable_support").
Should use wxGetApp().preset_bundle->full_config() instead.
Jira: STUDIO-3264
2. Allow different object having different brim_width to encode support's brim width.
In this way the arranging logic is simpler and more objects can be arranged in a plate.

Change-Id: Ic7178652147b0519d50b2eb9ceb214b4ef4c0251
(cherry picked from commit 0f163d59b40009cd29b8b1e32f730258ad0786dc)

src/libslic3r/ModelArrange.cpp

@@ -157,6 +157,11 @@ ArrangePolygon get_instance_arrange_poly(ModelInstance* instance, const Slic3r::
     }        
 #else
     ap.brim_width = 0;
+    // For by-layer printing, need to shrink bed a little, so the support won't go outside bed.
+    // We set it to 5mm because that's how much a normal support will grow by default.
+    auto supp_type_ptr = obj->get_config_value<ConfigOptionBool>(config, "enable_support");
+    if (supp_type_ptr && supp_type_ptr->getBool())
+        ap.brim_width = 5.0;
 #endif
     
     ap.height = obj->bounding_box().size().z();