deps/media-playback: Fix audio segment duration calc

With certain audio encoders, gaps can be introduced into the audio
packets, causing the audio duration to be miscalculated because it
calculated audio duration based upon the PTS of the current packet to
the last packet. However, this audio encoder also did not store
timestamps for most of its audio packets, causing PTS values to be
calculated based upon duration values. So those two things combined
caused audio timestamps to go all out of whack when playing back certain
videos with the media source. This is particularly prevalent with WMV
files using Microsoft codecs.

So to fix it, when the duration needs to be calculated, just calculate
the duration based upon the sample count of the audio packet. This fixes
the issue with the video in question that caused problems, and likely
fixes issues in a lot of videos that may have been floating out there
for some time. Basically this is a fix for a potential long-standing
issue.

Closes obsproject/obs-studio#3683

deps/media-playback/media-playback/decode.c

@@ -246,14 +246,14 @@ void mp_decode_push_packet(struct mp_decode *decode, AVPacket *packet)
 static inline int64_t get_estimated_duration(struct mp_decode *d,
 					     int64_t last_pts)
 {
-	if (last_pts)
-		return d->frame_pts - last_pts;
-
 	if (d->audio) {
 		return av_rescale_q(d->in_frame->nb_samples,
 				    (AVRational){1, d->in_frame->sample_rate},
 				    (AVRational){1, 1000000000});
 	} else {
+		if (last_pts)
+			return d->frame_pts - last_pts;
+
 		if (d->last_duration)
 			return d->last_duration;