Make support material generation idempotent

lib/Slic3r/Print/Object.pm

@@ -1005,8 +1005,8 @@ sub combine_infill {
 sub generate_support_material {
     my $self = shift;
     
-    # TODO: make this method idempotent by removing all support layers
-    # before checking whether we need to generate support or not
+    $self->clear_support_layers;
+    
     return unless ($self->config->support_material || $self->config->raft_layers > 0)
         && scalar(@{$self->layers}) >= 2;