lavc/qsvenc: set HRD buffer size

Hypothetical Reference Decoding (HRD) model assumes that data flows
into a buffer of the fixed size BufferSizeInKB with a constant bitrate.
Smaller BufferSizeInKB means smaller frame size variations,
but more difficult to maintain HRD.
Signed-off-by: Zhong Li <zhong.li@intel.com>
Signed-off-by: Maxym Dmytrychenko <maxim.d33@gmail.com>

libavcodec/qsvenc.c

@@ -503,6 +503,7 @@ static int init_video_param(AVCodecContext *avctx, QSVEncContext *q)
 #if QSV_HAVE_VCM
     case MFX_RATECONTROL_VCM:
 #endif
+        q->param.mfx.BufferSizeInKB   = avctx->rc_buffer_size / 8000;
         q->param.mfx.InitialDelayInKB = avctx->rc_initial_buffer_occupancy / 1000;
         q->param.mfx.TargetKbps       = avctx->bit_rate / 1000;
         q->param.mfx.MaxKbps          = avctx->rc_max_rate / 1000;