vd_lavc: request 1 more threads than CPU cores

This duplicates the logic which FFmpeg's libavcodec uses. The effects
are unknown, though it's somewhat clear that a single thread doesn't
necessarily saturate a single CPU.

(Eventually we should just let FFmpeg auto-init the thread count, but
for now I prefer it this way, so e.g. verbose mode will print the
thread count.)

common/av_common.c

@@ -135,6 +135,8 @@ void mp_set_avcodec_threads(struct mp_log *l, AVCodecContext *avctx, int threads
             threads = 1;
         } else {
             mp_verbose(l, "Detected %d logical cores.\n", threads);
+            if (threads > 1)
+                threads += 1; // extra thread for better load balancing
         }
         // Apparently some libavcodec versions have or had trouble with more
         // than 16 threads, and/or print a warning when using > 16.