fix bug in parsing of IRT WebVTT sequence (stts flattening when variable framerate is used)
fix bug in parsing of IRT WebVTT sequence (stts flattening when variable
framerate is used)

gpac/mp4box.js,gpac/mp4box.js,jhiesey/mp4box.js,sjames1958gm/mp4box.js,emmanouil/mp4box.js-3d-old,hanzeil/mp4box.js,emmanouil/mp4box.js-3d-old,b123400/mp4box.js,sjames1958gm/mp4box.js,cconcolato/mp4box.js,b123400/mp4box.js,jhiesey/mp4box.js,cconcolato/mp4box.js,hanzeil/mp4box.js