renderDocument
fun renderDocument(script: String, timeMillis: Long, frameWidth: Int, frameHeight: Int, startMicros: Long = timeMillis * 1000, endMicros: Long = (timeMillis + 1) * 1000): SubtitleCue.Bitmap?
Renders script at timeMillis into bitmap regions for a frameWidth x frameHeight frame. Returns null when nothing is visible there. The script is parsed once and kept until a different one arrives.