custom shaka TextDisplayer interface

[byWhish]

Hi i'm using TTML subtitles image format. There is a way to implemet in the receiver my own TextDisplayer interface like in the issue below.

shaka-project/shaka-player#2182

Thanks

[c0d3base]

CAF converts TTML to WebVTT. We do this because TTML has timing issues when rendering on the screen. We don't advice partners to build their own text UI.