字幕是ass的可修改 AlphaLevel 值, AlphaLevel取值0-255, 0不透明，255 全透明. 位置在下面例子中的红字处。
[V4 Styles]
Format: Name, Fontname, Fontsize, PrimaryColour, SecondaryColour, TertiaryColour, BackColour, Bold, Italic, BorderStyle, Outline, Shadow, Alignment, MarginL, MarginR, MarginV, AlphaLevel, Encoding
Style: Default,宋体,16,&H00ffff,&H00ffff,&H000000,&H000000,0,0,1,2,0,2,30,30,15,50,134
