Add a native widget to render ass subtitles.

[peerless2012]

With this commit #31, we can know the current video presentation time and when it's changed, So add a widget above video surface view and render ass subtitles on it can be possible.

Here are some new features:

• Support animation in ass, it's runtime render.
• Support HDR/DV, because it not use effects (which will broke HDR/DV).