lavc/x265: set preferred_transfer_characteristics for HLG

"HEVC HDR UHDTV Bitstreams using HLG10 shall also contain the
alternative_transfer_characteristics SEI message. The
alternative_transfer_characteristics SEI message shall be inserted on
the HEVC DVB_RAP, and preferred_transfer_characteristics shall be set
equal to "18", indicating Recommendation ITU-R BT. 2100 [45] HLG
system."
Signed-off-by: Zhong Li <zhongli_dev@126.com>

libavcodec/libx265.c

@@ -159,6 +159,10 @@ static av_cold int libx265_encode_init(AVCodecContext *avctx)
         // x265 validates the parameters internally
         ctx->params->vui.colorPrimaries          = avctx->color_primaries;
         ctx->params->vui.transferCharacteristics = avctx->color_trc;
+#if X265_BUILD >= 159
+        if (avctx->color_trc == AVCOL_TRC_ARIB_STD_B67)
+            ctx->params->preferredTransferCharacteristics = ctx->params->vui.transferCharacteristics;
+#endif
         ctx->params->vui.matrixCoeffs            = avctx->colorspace;
     }