video: do not show decoder framedrops if they're not requested

libavcodec makes it impossible to distinguish dropped frames (requested
with AVCodecContext.skip_frame), and cases when the decoder simply does
not return a frame by default (such as with VP9, which has invisible
reference frames).

This confuses users when decoding VP9 video. It's basically a cosmetic
issue, so just paint it over by ignoring them if framedropping is
disabled.

DOCS/man/mpv.rst

@@ -517,10 +517,9 @@ listed.
   rendering is too slow. Also can be incremented on "hiccups" and when the video
   frame couldn't be displayed on time. (``vo-drop-frame-count`` property.)
   If the decoder drops frames, the number of decoder-dropped frames is appended
-  to the display as well, e.g.: ``Dropped: 4/34``. This should almost never
-  happen, unless decoder-framedropping is enabled with one of the
-  ``--framedrop`` options, the stream contains errors, or a weird codec is in
-  use. (``drop-frame-count`` property.)
+  to the display as well, e.g.: ``Dropped: 4/34``. This happens only if
+  decoder-framedropping is enabled with the ``--framedrop`` options.
+  (``drop-frame-count`` property.)
 - Cache state, e.g. ``Cache:  2s+134KB``. Visible if the stream cache is enabled.
   The first value shows the amount of video buffered in the demuxer in seconds,
   the second value shows *additional* data buffered in the stream cache in

player/video.c

@@ -405,7 +405,8 @@ static int decode_image(struct MPContext *mpctx)
     talloc_free(pkt);
 
     if (had_packet && !d_video->waiting_decoded_mpi &&
-        mpctx->video_status == STATUS_PLAYING)
+        mpctx->video_status == STATUS_PLAYING &&
+        (mpctx->opts->frame_dropping & 2))
     {
         mpctx->dropped_frames_total++;
         mpctx->dropped_frames++;