lavf: add AV_DISPOSITION flags for WebVTT text track kinds

There are 4 separate WebVTT text track kinds: subtitles (the default
if not otherwise specified), captions, descriptions, and metadata.
The WebM muxer needs to know which WebVTT text track kind this is, in
order to synthesize the correct track type and codec id.

To allow a demuxer to indicate the text track kind of the input, a new
set of AV_DISPOSITION flag values has been added, corresponding to
each of the non-default text track kind values.

doc/APIchanges

@@ -18,6 +18,9 @@ API changes, most recent first:
 2013-06-xx - xxxxxxx - lavc 55.10.0 - avcodec.h
   Add MPEG-2 AAC profiles
 
+2013-06-xx - xxxxxxx - lavf 55.10.100 - avformat.h
+  Add AV_DISPOSITION_* flags to indicate text track kind.
+
 2013-06-xx - xxxxxxx - lavu 52.36.100
   Add AVRIPEMD:
    av_ripemd_alloc()

libavformat/avformat.h

@@ -635,6 +635,13 @@ typedef struct AVIndexEntry {
  */
 #define AV_DISPOSITION_ATTACHED_PIC      0x0400
 
+/**
+ * To specify text track kind (different from subtitles default).
+ */
+#define AV_DISPOSITION_CAPTIONS     0x10000
+#define AV_DISPOSITION_DESCRIPTIONS 0x20000
+#define AV_DISPOSITION_METADATA     0x40000
+
 /**
  * Options for behavior on timestamp wrap detection.
  */

libavformat/version.h

@@ -30,7 +30,7 @@
 #include "libavutil/avutil.h"
 
 #define LIBAVFORMAT_VERSION_MAJOR 55
-#define LIBAVFORMAT_VERSION_MINOR  9
+#define LIBAVFORMAT_VERSION_MINOR 10
 #define LIBAVFORMAT_VERSION_MICRO 100
 
 #define LIBAVFORMAT_VERSION_INT AV_VERSION_INT(LIBAVFORMAT_VERSION_MAJOR, \