flvdec: read index stored in the 'keyframes' tag.

'keyframes' metatag is not part of the standard, it is just
convention to use such kind of metatag information for indexing.
Structure is following, it allows to have it inconsistent:
keyframes:
 times (array):
  time0 (num)
  time1 (num)
  time2 (num)
 filepositions (array)
  position0 (num)
  position1 (num)
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavformat/flvdec.c

@@ -31,6 +31,10 @@
 #include "avio_internal.h"
 #include "flv.h"
 
+#define KEYFRAMES_TAG            "keyframes"
+#define KEYFRAMES_TIMESTAMP_TAG  "times"
+#define KEYFRAMES_BYTEOFFSET_TAG "filepositions"
+
 typedef struct {
     int wrong_dts; ///< wrong dts due to negative cts
 } FLVContext;
@@ -125,6 +129,64 @@ static int amf_get_string(AVIOContext *ioc, char *buffer, int buffsize) {
     return length;
 }
 
+static int parse_keyframes_index(AVFormatContext *s, AVIOContext *ioc, AVStream *vstream, int64_t max_pos) {
+    unsigned int arraylen = 0, timeslen = 0, fileposlen = 0, i;
+    double num_val;
+    char str_val[256];
+    int64_t *times = NULL;
+    int64_t *filepositions = NULL;
+    int ret = 0;
+
+    while (avio_tell(ioc) < max_pos - 2 && amf_get_string(ioc, str_val, sizeof(str_val)) > 0) {
+        int64_t* current_array;
+
+        // Expect array object in context
+        if (avio_r8(ioc) != AMF_DATA_TYPE_ARRAY)
+            break;
+
+        arraylen = avio_rb32(ioc);
+        /*
+         * Expect only 'times' or 'filepositions' sub-arrays in other case refuse to use such metadata
+         * for indexing
+         */
+        if (!strcmp(KEYFRAMES_TIMESTAMP_TAG, str_val) && !times) {
+            if (!(times = av_mallocz(sizeof(*times) * arraylen))) {
+                ret = AVERROR(ENOMEM);
+                goto finish;
+            }
+            timeslen = arraylen;
+            current_array = times;
+        } else if (!strcmp(KEYFRAMES_BYTEOFFSET_TAG, str_val) && !filepositions) {
+            if (!(filepositions = av_mallocz(sizeof(*filepositions) * arraylen))) {
+                ret = AVERROR(ENOMEM);
+                goto finish;
+            }
+            fileposlen = arraylen;
+            current_array = filepositions;
+        } else // unexpected metatag inside keyframes, will not use such metadata for indexing
+            break;
+
+        for (i = 0; i < arraylen && avio_tell(ioc) < max_pos - 1; i++) {
+            if (avio_r8(ioc) != AMF_DATA_TYPE_NUMBER)
+                goto finish;
+            num_val = av_int2dbl(avio_rb64(ioc));
+            current_array[i] = num_val;
+        }
+    }
+
+    if (timeslen == fileposlen)
+         for(i = 0; i < arraylen; i++)
+             av_add_index_entry(vstream, filepositions[i], times[i]*1000, 0, 0, AVINDEX_KEYFRAME);
+    else
+        av_log(s, AV_LOG_WARNING, "Invalid keyframes object, skipping.\n");
+
+finish:
+    av_freep(&times);
+    av_freep(&filepositions);
+    avio_seek(ioc, max_pos, SEEK_SET);
+    return ret;
+}
+
 static int amf_parse_object(AVFormatContext *s, AVStream *astream, AVStream *vstream, const char *key, int64_t max_pos, int depth) {
     AVCodecContext *acodec, *vcodec;
     AVIOContext *ioc;
@@ -149,6 +211,10 @@ static int amf_parse_object(AVFormatContext *s, AVStream *astream, AVStream *vst
         case AMF_DATA_TYPE_OBJECT: {
             unsigned int keylen;
 
+            if (!strcmp(KEYFRAMES_TAG, key) && depth == 1)
+                if (parse_keyframes_index(s, ioc, vstream, max_pos) < 0)
+                    return -1;
+
             while(avio_tell(ioc) < max_pos - 2 && (keylen = avio_rb16(ioc))) {
                 avio_skip(ioc, keylen); //skip key string
                 if(amf_parse_object(s, NULL, NULL, NULL, max_pos, depth + 1) < 0)