common/lwlibav_video.c: fix incorrect seeking in some mkvs, again

It turns out not only the first frame could have invalid DTS, there
are mkv files in the wild that contains first few frames with an
invalid DTS. It's beyond me how mkvmerge could have created such a
file given that mkv are supposed to be seeking by DTS.

Next time, if you face such a mkv, you can:
1. mkvextract the video and then remux with mkvmerge to force it to
   recreate cue points for all IDR..
2. add `--cue 0:all` to mkvmerge and remux the broken mkv to fix it.

Fixes #14.

Signed-off-by: akarin <[email protected]>

common/lwlibav_video.c

@@ -427,7 +427,10 @@ static uint32_t correct_current_frame_number
     uint32_t                        goal
 )
 {
-#define MATCH_DTS( j ) (info[j].dts == pkt->dts)
+// It's possible that the first few encoded frames all have DTS AV_NOPTS_VALUE, so we really
+// shouldn't stop when dts matches: at least we should fallback to checking POS if allowed.
+// Also note that `j` might contain side-effects, must always evaluate it exactly once!
+#define MATCH_DTS( j ) (info[j].dts == pkt->dts && (pkt->dts != AV_NOPTS_VALUE || ((vdhp->lw_seek_flags & SEEK_POS_CORRECTION) == 0)))
 #define MATCH_POS( j ) ((vdhp->lw_seek_flags & SEEK_POS_CORRECTION) && info[j].file_offset == pkt->pos)
     order_converter_t  *oc   = vdhp->order_converter;
     video_frame_info_t *info = vdhp->frame_list;