vp9: set skip flag if the block had no coded coefficients.

This reproduces libvpx behaviour. It seems like it originally only
targeted loopfilter behaviour, but this unfortunately effects following
block contexting and thus directs bitstream sync.

libavcodec/vp9.c

@@ -2298,7 +2298,7 @@ static int decode_coeffs_b32_16bpp(VP9Context *s, int16_t *coef, int n_coeffs,
                                    nnz, scan, nb, band_counts, qmul);
 }
 
-static av_always_inline void decode_coeffs(AVCodecContext *ctx, int is8bitsperpixel)
+static av_always_inline int decode_coeffs(AVCodecContext *ctx, int is8bitsperpixel)
 {
     VP9Context *s = ctx->priv_data;
     VP9Block *b = s->b;
@@ -2327,6 +2327,7 @@ static av_always_inline void decode_coeffs(AVCodecContext *ctx, int is8bitsperpi
     const int16_t *y_band_counts = band_counts[b->tx];
     const int16_t *uv_band_counts = band_counts[b->uvtx];
     int bytesperpixel = is8bitsperpixel ? 1 : 2;
+    int total_coeff = 0;
 
 #define MERGE(la, end, step, rd) \
     for (n = 0; n < end; n += step) \
@@ -2346,6 +2347,7 @@ static av_always_inline void decode_coeffs(AVCodecContext *ctx, int is8bitsperpi
                                      c, e, p, a[x] + l[y], yscans[txtp], \
                                      ynbs[txtp], y_band_counts, qmul[0]); \
             a[x] = l[y] = !!res; \
+            total_coeff |= !!res; \
             if (step >= 4) { \
                 AV_WN16A(&s->eob[n], res); \
             } else { \
@@ -2419,6 +2421,7 @@ static av_always_inline void decode_coeffs(AVCodecContext *ctx, int is8bitsperpi
                                      16 * step * step, c, e, p, a[x] + l[y], \
                                      uvscan, uvnb, uv_band_counts, qmul[1]); \
             a[x] = l[y] = !!res; \
+            total_coeff |= !!res; \
             if (step >= 4) { \
                 AV_WN16A(&s->uveob[pl][n], res); \
             } else { \
@@ -2458,16 +2461,18 @@ static av_always_inline void decode_coeffs(AVCodecContext *ctx, int is8bitsperpi
             break;
         }
     }
+
+    return total_coeff;
 }
 
-static void decode_coeffs_8bpp(AVCodecContext *ctx)
+static int decode_coeffs_8bpp(AVCodecContext *ctx)
 {
-    decode_coeffs(ctx, 1);
+    return decode_coeffs(ctx, 1);
 }
 
-static void decode_coeffs_16bpp(AVCodecContext *ctx)
+static int decode_coeffs_16bpp(AVCodecContext *ctx)
 {
-    decode_coeffs(ctx, 0);
+    return decode_coeffs(ctx, 0);
 }
 
 static av_always_inline int check_intra_mode(VP9Context *s, int mode, uint8_t **a,
@@ -3180,10 +3185,17 @@ static void decode_b(AVCodecContext *ctx, int row, int col,
                            (s->ss_v && h4 * 2 == (1 << b->tx)));
 
         if (!b->skip) {
+            int has_coeffs;
+
             if (bytesperpixel == 1) {
-                decode_coeffs_8bpp(ctx);
+                has_coeffs = decode_coeffs_8bpp(ctx);
             } else {
-                decode_coeffs_16bpp(ctx);
+                has_coeffs = decode_coeffs_16bpp(ctx);
+            }
+            if (!has_coeffs && b->bs <= BS_8x8 && !b->intra) {
+                b->skip = 1;
+                memset(&s->above_skip_ctx[col], 1, w4);
+                memset(&s->left_skip_ctx[s->row7], 1, h4);
             }
         } else {
             int row7 = s->row7;