Fix for ticket #1559

sh_no_output_of_prior_pics_flag is unconditionally overridden here.

According to Spec this allowed (but not mandatory) only if one of
critical parameters has changed: PicWidthMaxInSamplesY,
PicHeightMaxInSamplesY, MaxChromaFormat, MaxBitDepthMinus8, or
dpb_max_dec_pic_buffering_minus1.