vp8: make wait/thread_mb_pos atomic.

Fixes tsan warnings like this in fate-vp8-test-vector-007:

WARNING: ThreadSanitizer: data race (pid=3590)
  Write of size 4 at 0x7d8c0000e07c by thread T2:
    #0 decode_mb_row_no_filter src/libavcodec/vp8.c:2330 (ffmpeg+0x000000ffb59e)
[..]
  Previous write of size 4 at 0x7d8c0000e07c by thread T1:
    #0 decode_mb_row_no_filter src/libavcodec/vp8.c:2330 (ffmpeg+0x000000ffb59e)

1 files changed, 4 insertions, 2 deletions

diff --git a/libavcodec/vp8.h b/libavcodec/vp8.h
index 3910b5c0dd..d7e7680276 100644
--- a/libavcodec/vp8.h
+++ b/libavcodec/vp8.h
@@ -26,6 +26,8 @@
 #ifndef AVCODEC_VP8_H
 #define AVCODEC_VP8_H
 
+#include <stdatomic.h>
+
 #include "libavutil/buffer.h"
 #include "libavutil/thread.h"
 
@@ -114,8 +116,8 @@ typedef struct VP8ThreadData {
     pthread_mutex_t lock;
     pthread_cond_t cond;
 #endif
-    int thread_mb_pos; // (mb_y << 16) | (mb_x & 0xFFFF)
-    int wait_mb_pos; // What the current thread is waiting on.
+    atomic_int thread_mb_pos; // (mb_y << 16) | (mb_x & 0xFFFF)
+    atomic_int wait_mb_pos; // What the current thread is waiting on.
 
 #define EDGE_EMU_LINESIZE 32
     DECLARE_ALIGNED(16, uint8_t, edge_emu_buffer)[21 * EDGE_EMU_LINESIZE];