lavc/pgssubdec: use the PTS from the presentation segment.

According to the sample for trac ticket #1722, PGS subtitles
are decoded from several packets at the same DTS and varying PTS.
The PTS from the presentation segment seem to be the valid one;
in particular, clear subtitles are too early with the other PTS.

libavcodec/pgssubdec.c

@@ -67,6 +67,7 @@ typedef struct PGSSubContext {
     PGSSubPresentation presentation;
     uint32_t           clut[256];
     PGSSubPicture      pictures[UINT16_MAX];
+    int64_t            pts;
     int forced_subs_only;
 } PGSSubContext;
 
@@ -378,6 +379,7 @@ static int display_end_segment(AVCodecContext *avctx, void *data,
 {
     AVSubtitle    *sub = data;
     PGSSubContext *ctx = avctx->priv_data;
+    int64_t pts;
 
     uint16_t rect;
 
@@ -387,7 +389,10 @@ static int display_end_segment(AVCodecContext *avctx, void *data,
      *      not been cleared by a subsequent empty display command.
      */
 
+    pts = ctx->pts != AV_NOPTS_VALUE ? ctx->pts : sub->pts;
     memset(sub, 0, sizeof(*sub));
+    sub->pts = pts;
+    ctx->pts = AV_NOPTS_VALUE;
 
     // Blank if last object_count was 0.
     if (!ctx->presentation.object_count)
@@ -436,8 +441,10 @@ static int display_end_segment(AVCodecContext *avctx, void *data,
 static int decode(AVCodecContext *avctx, void *data, int *data_size,
                   AVPacket *avpkt)
 {
+    PGSSubContext *ctx = avctx->priv_data;
     const uint8_t *buf = avpkt->data;
     int buf_size       = avpkt->size;
+    AVSubtitle *sub    = data;
 
     const uint8_t *buf_end;
     uint8_t       segment_type;
@@ -482,6 +489,7 @@ static int decode(AVCodecContext *avctx, void *data, int *data_size,
             break;
         case PRESENTATION_SEGMENT:
             parse_presentation_segment(avctx, buf, segment_length);
+            ctx->pts = sub->pts;
             break;
         case WINDOW_SEGMENT:
             /*