IVF是一个非常简单的视频容器。用于封装VP8/VP9的数据。下面多字节的数据是采用小端编码(little-endian)。IVF文件头由32字节组成：bytes 0-3
signature: 'DKIF'bytes 4-5
version (should be 0)bytes 6-7
length of header in bytesbytes 8-11 codec FourCC (e.g., 'VP80')bytes 12-13width in pixelsbytes 14-15height in pixelsbytes 16-19frame ratebytes 20-23time scalebytes 24-27number of frames in filebytes 28-31unused文件后跟随多帧的数据，第帧数据由一个12字节的帧头组成。bytes 0-3
size of frame in bytes (not including the 12-byte header)bytes 4-11 64-bit presentation timestampbytes 12.. frame data
