player: better handling of video with no timestamps

Trying to handle such video is almost worthless, but it was requested by
at least 2 users.

If there are no timestamps, enable byte seeking by setting
ts_resets_possible. Use the video FPS (wherever it comes from) and the
audio samplerate for timing. The latter was already done by making the
first packet emit DTS=0; remove this again and do it "properly" in a
higher level.

audio/decode/dec_audio.c

@@ -164,6 +164,9 @@ static int decode_new_frame(struct dec_audio *da)
         if (ret < 0)
             return ret;
 
+        if (da->pts == MP_NOPTS_VALUE && da->header->missing_timestamps)
+            da->pts = 0;
+
         if (da->waiting) {
             da->pts_offset += da->waiting->samples;
             da->decode_format = *da->waiting;