Implementing full support for 3GPP Timed Text Subtitles

by Niklesh Lalwani for FFmpeg

The standard subtitle format used in MP4 containers is 3GPP timed text. It is the only subtitle format supported in Apple's media players on OS X and iOS. As such it is important for FFmpeg to support the format as fully as possible. Currently, it supports only a limited subset of the format. For this project, the goal would be to implement complete support for these features and have the implementation fully reviewed and merged into FFmpeg.