av1_init_scan_order
initialize data structures related to adaptive scan order

av1_update_scan_prob
update nonzero probabilities from nonzero counts

av1_augment_prob
embed r + c and coeff_idx info with nonzero probabilities.
When sorting the nonzero probabilities, if there is a tie,
the coefficient with smaller r + c will be scanned first

av1_update_sort_order
apply quick sort on nonzero probabilities to obtain a sort order

av1_update_scan_order
apply topological sort on the nonzero probabilities sorting order to
guarantee each to-be-scanned coefficient's upper and left coefficient
will be scanned before the to-be-scanned coefficient.

av1_update_neighbors
For each coeff_idx in scan[], update its above and left neighbors in
neighbors[] accordingly.

Change-Id: I64c4938057daf8e30e48609a00ecc08d2e3062f4