h264: correct border check.

When backing up the top-left border, check that the top-left
(rather than left) MB indeed does belong to our slice. If it
doesn't, backing up has no positive effect but may accidentally
interfere with other threads writing in the same space.

Fixes occasional one-off effects when enabling slice-MT.

libavcodec/h264.c

@@ -1034,7 +1034,7 @@ static inline void xchg_mb_border(H264Context *h, uint8_t *src_y,
                                   int linesize, int uvlinesize,
                                   int xchg, int simple, int pixel_shift){
     MpegEncContext * const s = &h->s;
-    int deblock_left;
+    int deblock_topleft;
     int deblock_top;
     int top_idx = 1;
     uint8_t *top_border_m1;
@@ -1050,11 +1050,11 @@ static inline void xchg_mb_border(H264Context *h, uint8_t *src_y,
     }
 
     if(h->deblocking_filter == 2) {
-        deblock_left = h->left_type[0];
-        deblock_top  = h->top_type;
+        deblock_topleft = h->slice_table[h->mb_xy - 1 - s->mb_stride] == h->slice_num;
+        deblock_top     = h->top_type;
     } else {
-        deblock_left = (s->mb_x > 0);
-        deblock_top =  (s->mb_y > !!MB_FIELD);
+        deblock_topleft = (s->mb_x > 0);
+        deblock_top     = (s->mb_y > !!MB_FIELD);
     }
 
     src_y  -=   linesize + 1 + pixel_shift;
@@ -1077,7 +1077,7 @@ if (xchg) AV_SWAP64(b,a);\
 else      AV_COPY64(b,a);
 
     if(deblock_top){
-        if(deblock_left){
+        if(deblock_topleft){
             XCHG(top_border_m1 + (8 << pixel_shift), src_y - (7 << pixel_shift), 1);
         }
         XCHG(top_border + (0 << pixel_shift), src_y + (1 << pixel_shift), xchg);
@@ -1088,7 +1088,7 @@ else      AV_COPY64(b,a);
     }
     if(simple || !CONFIG_GRAY || !(s->flags&CODEC_FLAG_GRAY)){
         if(deblock_top){
-            if(deblock_left){
+            if(deblock_topleft){
                 XCHG(top_border_m1 + (16 << pixel_shift), src_cb - (7 << pixel_shift), 1);
                 XCHG(top_border_m1 + (24 << pixel_shift), src_cr - (7 << pixel_shift), 1);
             }
@@ -2611,7 +2611,7 @@ static void loop_filter(H264Context *h, int start_x, int end_x){
         }
     }
     h->slice_type= old_slice_type;
-    s->mb_x= 0;
+    s->mb_x= end_x;
     s->mb_y= end_mb_y - FRAME_MBAFF;
     h->chroma_qp[0] = get_chroma_qp(h, 0, s->qscale);
     h->chroma_qp[1] = get_chroma_qp(h, 1, s->qscale);