player: smooth out frame durations by averaging them

Helps somewhat with muxer-rounded timestamps.

There is some danger that this introduces a timestamp drift. But since
they are averaged values (unlike as when using an incorrect container
framerate hint), any potential drift shouldn't be too brutal, or
compensate itself soon. So I won't bother yet with comparing the results
with the real timestamp, unless we run into actual problems.

Of course we still prefer potentially real timestamps over the
approximated ones. But unless the timestamps match the container FPS,
we can't know whether they are (no, checking whether the they have
microsecond components would be cheating). Perhaps in future, we could
let the demuxer export the timebase - if the timebase is not 1000 (or
divisible by it), we know that millisecond-rounded timestamps won't
happen.

player/video.c

@@ -1058,9 +1058,9 @@ static void calculate_frame_duration(struct MPContext *mpctx)
         total += dur;
         num_dur += 1;
     }
+    double approx_duration = num_dur > 0 ? total / num_dur : duration;
 
     // Try if the demuxer frame rate fits - if so, just take it.
-    double approx_duration = duration;
     if (demux_duration > 0) {
         // Note that even if each timestamp is within rounding tolerance, it
         // could literally not add up (e.g. if demuxer FPS is rounded itself).