Disable timestamp shifting in subtitle-only MKV files

For normal media file, the starting timestamp is shifted to 0. This is necessary in order to playback trimmed audio/video that has a non-zero starting timestamp.
However, if the file only contains subtitle, we do not want to shift the start time of first line to 0. Because subtitles are never played alone, but usually attached to an audio/video. There is no way to get the starting timestamp of the audio/video in a subtitle-only MKV, so assuming 0 is a safe guess.

Fix TypesettingTools#91

src/MatroskaParser.c

@@ -34,6 +34,11 @@
 
  */
 
+// WARNING: This library is patched by Aegisub to NOT shift timestamps. The modified parts are included in #ifdef AEGISUB blocks
+// We are only using this library to extract subtitles from MKV files. The patches may break reading other tracks.
+// Do NOT use the modified version to read non-subtitle tracks!
+#define AEGISUB
+
 #include <stdlib.h>
 #include <stdarg.h>
 #include <stdio.h>
@@ -2720,12 +2725,29 @@ static void parseFile(MatroskaFile *mf) {
   if (!mf->seen.Cluster)
     mf->pCluster = mf->pSegmentTop;
 
+#ifdef AEGISUB
+  // In a subtitle-only MKV, the start time of first block is meaningful. Therefore do not shift offset=(-firstTimecode) globally
+  // By setting firstTimecode to 0, timestamps will not be shifted during parsing.
+  int subtitle_only = 1;
+  for (unsigned tracknum=0;tracknum<mf->nTracks;++tracknum)
+    if (mf->Tracks[tracknum]->Type != TT_SUB) {
+      subtitle_only = 0;
+      break;
+    }
+  if (subtitle_only) {
+    mf->firstTimecode = 0;
+  }
+  else {
+#endif
   adjust = mf->firstTimecode * mf->Seg.TimecodeScale;
 
   for (i=0;i<mf->nChapters;++i)
     fixupChapter(adjust, &mf->Chapters[i]);
 
   fixupCues(mf);
+#ifdef AEGISUB
+  }
+#endif
 
   // release extra memory
   ARELEASE(mf,mf,Tracks);