lavf: add AVFMT_SEEK_TO_PTS to indicate seeking is per PTS.

See: [FFmpeg-devel] [PATCH] Add documentation that seeking is done by DTS and not PTS
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/avformat.h

@@ -351,6 +351,7 @@ typedef struct AVProbeData {
 #define AVFMT_NOGENSEARCH   0x4000 /**< Format does not allow to fallback to generic search */
 #define AVFMT_NO_BYTE_SEEK  0x8000 /**< Format does not allow seeking by bytes */
 #define AVFMT_ALLOW_FLUSH  0x10000 /**< Format allows flushing. If not set, the muxer will not receive a NULL packet in the write_packet function. */
+#define AVFMT_SEEK_TO_PTS   0x4000000 /**< Seeking is based on PTS */
 #define AVFMT_TS_NONSTRICT  0x8000000 /**< Format does not require strictly
                                            increasing timestamps, but they must
                                            still be monotonic */