[demuxers/Matroska] Preliminarily mark index entries as keyframes bas…

…ed on Simple Block headers

This should help to deal with some broken files where we cannot locate the
Cues element and the video codec is not H.264, HEVC, MPEG-4 or MPEG-2
where we ignore both flags and Cues and rely only on frame headers.

avidemux_plugins/ADM_demuxers/Matroska/ADM_mkvIndexer.cpp

@@ -120,8 +120,6 @@ uint8_t mkvHeader::indexLastCluster(ADM_ebml_file *parser)
 */
 uint8_t mkvHeader::indexBlock(ADM_ebml_file *parser,uint32_t len,uint32_t clusterTimeCodeMs)
 {
-    int entryFlags=0;
-
     uint64_t tail=parser->tell()+len;
     // Read Track id
     uint32_t tid=parser->readEBMCode();
@@ -141,6 +139,8 @@ uint8_t mkvHeader::indexBlock(ADM_ebml_file *parser,uint32_t len,uint32_t cluste
 
     //int lacing=((flags>>1)&3);
 
+    uint32_t entryFlags = (flags & 0x80)? AVI_KEY_FRAME : 0;
+
     addIndexEntry(track,parser,blockBegin,tail-blockBegin,entryFlags,clusterTimeCodeMs+timecode);
     parser->seek(tail);
     return 1;
@@ -188,7 +188,7 @@ uint8_t mkvHeader::addIndexEntry(uint32_t track,ADM_ebml_file *parser,uint64_t w
 
     ix.pos=where;
     ix.size=size;
-    ix.flags=0;
+    ix.flags=flags;
     ix.Dts=timecodeMS*_timeBase;
     ix.Pts=timecodeMS*_timeBase;
     //printf("Track=%d, timecode=%d timeBase=%d, Pts=%d\n",track,(int)timecodeMS,(int)_timeBase,ix.Pts);