This expands on the work done in d2d8a229 and allows incremental sort
to be considered during create_window_paths().

Author: David Rowley
Reviewed-by: Daniel Gustafsson, Tomas Vondra
Discussion: https://postgr.es/m/CAApHDvoOHobiA2x13NtWnWLcTXYj9ddpCkv9PnAJQBMegYf_xw%40mail.gmail.com