Zhao Zhili 693c5be320 avformat/flvenc: fix timestamp of key frame index
Firstly, the timestamps generated from framerate are inaccurate for
variable framerate mode.

Secondly, the timestamps always start from zero, while pts/dts can
start from nonzero. FLV demuxer rejects such index with message:
"Found invalid index entries, clearing the index".
2022-08-03 17:52:45 +08:00
..
2022-06-01 14:56:50 +02:00