h264: copy pixel_shift between slice threading contexts.

Fixes "make THREADS=2 THREAD_TYPE=2
fate-h264-conformance-frext-pph10i3_panasonic_a".

libavcodec/h264.c

@@ -1965,6 +1965,7 @@ static int decode_slice_header(H264Context *h, H264Context *h0){
             c->h264dsp = h->h264dsp;
             c->sps = h->sps;
             c->pps = h->pps;
+            c->pixel_shift = h->pixel_shift;
             init_scan_tables(c);
             clone_tables(c, h, i);
         }