Improved "ensure vertical wall thickness" feature for multi material:

If the "interface_shells" feature is disabled, the "ensure vertical wall thickness"
feature will not add full infill over internal shells.

Fixed some issues with delayed loading of 3d scenes.

lib/Slic3r/GUI/Plater/3DPreview.pm

@@ -298,7 +298,7 @@ sub set_number_extruders {
 # Called by the Platter wxNotebook when this page is activated.
 sub OnActivate {
     my ($self) = @_;
-    $self->reload_scene(1) if ($self->{reload_delayed});
+    $self->reload_print(1) if ($self->{reload_delayed});
 }
 
 1;