The final parameter of check_available determines whether the motion
estimation region constraints imposed in section 8.5.2.3 of H.266 (V3)
on MVP candidates apply to the current candidate or not. In the case of
IBC spatial merge candidates they do not, as their availability is
dependent only on the criteria described in sections 8.6.2.3 and 6.4.4,
which do not include this constraint on the motion estimation region.
Signed-off-by: Frank Plowman <post@frankplowman.com>