In gimp_layer_start_move(), keep the set of ancestors for which for
which we suspended mask cropping, so that we can resume mask
cropping for the same groups in gimp_layer_end_move().  This is
necessary, since gimp_image_remove_layer() calls gimp_item
start_move() before removing the layer from the layer tree, and
gimp_item_end_move() after removing the layer from the layer tree,
at which point the layer has no ancestors.