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)
