MP4入门知识，MP4 文件格式中，所有的内容存在一个称为 movie 的容器中。一个 movie 可以由多个 tracks 组成。每个 track 就是一个随时间变化的媒体序列-Getting knowledge MP4, MP4 file format, all content exists in a container called a movie. A movie can consist of multiple tracks. Each track is a sequence of time-varying media