h264: Copy h264chroma dsp context to slice thread copies

This fixes slice threading which seems to have been broken since
79dad2a9.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/h264.c

@@ -2569,6 +2569,7 @@ static int h264_slice_header_init(H264Context *h, int reinit)
             memset(&c->s + 1, 0, sizeof(H264Context) - sizeof(MpegEncContext));
             c->h264dsp     = h->h264dsp;
             c->h264qpel    = h->h264qpel;
+            c->h264chroma  = h->h264chroma;
             c->sps         = h->sps;
             c->pps         = h->pps;
             c->pixel_shift = h->pixel_shift;