avcodec/srtdec: Keep exact end times

When using FFmpeg as an SRT normalizer, the end timestamps were modified
compared to the original.

libavcodec/srtdec.c

@@ -202,7 +202,7 @@ static int srt_decode_frame(AVCodecContext *avctx,
 {
     AVSubtitle *sub = data;
     AVBPrint buffer;
-    int ts_start, ts_end, x1 = -1, y1 = -1, x2 = -1, y2 = -1;
+    int ts_start, ts_duration, x1 = -1, y1 = -1, x2 = -1, y2 = -1;
     int size, ret;
     const uint8_t *p = av_packet_get_side_data(avpkt, AV_PKT_DATA_SUBTITLE_POSITION, &size);
 
@@ -218,17 +218,21 @@ static int srt_decode_frame(AVCodecContext *avctx,
 
     av_bprint_init(&buffer, 0, AV_BPRINT_SIZE_UNLIMITED);
 
-        // TODO: reindent
-            // Do final divide-by-10 outside rescale to force rounding down.
-            ts_start = av_rescale_q(avpkt->pts,
-                                    avctx->time_base,
-                                    (AVRational){1,100});
-            ts_end   = av_rescale_q(avpkt->pts + avpkt->duration,
-                                    avctx->time_base,
-                                    (AVRational){1,100});
+    // Do final divide-by-10 outside rescale to force rounding down.
+    ts_start = av_rescale_q(avpkt->pts,
+                            avctx->time_base,
+                            (AVRational){1,100});
+
+    // Floor the duration (for ASS output)
+    ts_duration = avpkt->duration / 10;
+
+    // Set an exact end display time to prevent the rounding for ASS messing it up
+    sub->end_display_time = av_rescale_q(avpkt->duration,
+                                         avctx->pkt_timebase,
+                                         (AVRational){1,1000});
 
     srt_to_ass(avctx, &buffer, avpkt->data, x1, y1, x2, y2);
-    ret = ff_ass_add_rect_bprint(sub, &buffer, ts_start, ts_end-ts_start);
+    ret = ff_ass_add_rect_bprint(sub, &buffer, ts_start, ts_duration);
     av_bprint_finalize(&buffer, NULL);
     if (ret < 0)
         return ret;