avcodec/vp8: Change criterion for calling ff_thread_finish_setup()

The current criterion is to check for the existence of
update_thread_context. Change this to check for whether
we are actually decoding VP8 (and not VP7 or VP8-in-WebP).
This is equivalent to the current criterion, but allows
the WebP decoder to evolve and to get its own update_thread_context.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/vp8.c

@@ -2754,7 +2754,7 @@ int vp78_decode_frame(AVCodecContext *avctx, AVFrame *rframe, int *got_frame,
 
     s->next_framep[VP8_FRAME_CURRENT] = curframe;
 
-    if (ffcodec(avctx->codec)->update_thread_context)
+    if (!is_vp7 && !s->actually_webp)
         ff_thread_finish_setup(avctx);
 
     if (avctx->hwaccel) {