hevc: clip pixels when transquant bypass is used

The reconstructed picture should always be clipped (see section 8.6.5),
previously we did not clip coding units where
cu_transquant_bypass_flag == 1.

Sample-Id: 00001325-google
Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
