Adds color support for SimpleTextDisplayer and WebVttGenerator (only one
place to fix both now thanks to #4941).
It's limited to the [8 colors
classes](https://w3c.github.io/webvtt/#default-text-color) supported by
the WebVTT specification, and also works with their 3 or 6-digit hex
variants (if the stream has TTML subtitles).
It does not support rgb, rgba or any colors other than these 8.
Fixes#4545
---------
Co-authored-by: Alvaro Velad Galvan <ladvan91@hotmail.com>
Albin Larsson
Álvaro Velad Galván