vp8: update frame size changes on thread context switches.

This properly synchronizes frame size changes between threads if
subsequent threads abort decoding before frame size is initialized, i.e.
it prevents the thread after that from ping-ponging back to the original
value.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind

libavcodec/vp8.c

@@ -1833,6 +1833,8 @@ static int vp8_decode_update_thread_context(AVCodecContext *dst, const AVCodecCo
         (s_src->mb_width != s->mb_width || s_src->mb_height != s->mb_height)) {
         free_buffers(s);
         s->maps_are_invalid = 1;
+        s->mb_width  = s_src->mb_width;
+        s->mb_height = s_src->mb_height;
     }
 
     s->prob[0] = s_src->prob[!s_src->update_probabilities];