Let H.264 decoder skip deblocking across slices

if running with multiple threads and CODEC_FLAGS2_FAST is set.
Thus, it may decode the slices in parallel to gain speed.

Patch by Andreas Öman: [andreas olebyn nu]

Originally committed as revision 10431 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/h264.c

@@ -3815,6 +3815,11 @@ static int decode_slice_header(H264Context *h, H264Context *h0){
     }
 
     if(h->deblocking_filter == 1 && h0->max_contexts > 1) {
+        if(s->avctx->flags2 & CODEC_FLAG2_FAST) {
+            /* Cheat slightly for speed:
+               Dont bother to deblock across slices */
+            h->deblocking_filter = 2;
+        } else {
         h0->max_contexts = 1;
         if(!h0->single_decode_warning) {
             av_log(s->avctx, AV_LOG_INFO, "Cannot parallelize deblocking type 1, decoding such frames in sequential order\n");
@@ -3822,6 +3827,7 @@ static int decode_slice_header(H264Context *h, H264Context *h0){
         }
         if(h != h0)
             return 1; // deblocking switched inside frame
+        }
     }
 
     if(   s->avctx->skip_loop_filter >= AVDISCARD_ALL