There's a lag of one CTB line for SAO behind deblocking filter, except for
last line. However, once SAO has been completed on a line, all its pixels,
i.e. up to y+ctb_size are filtered and ready to be used as reference.

Without SAO, when deblocking filter finishes a CTB line, only the bottom
bottom 4 pixels may be filtered when next CTB is process by the deblocing.

The await_progess for hevc then checks whether the bottom pixels of a PU
requires access beyond that point, so the reporting should effectively
report up to the the above limits.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>