Rainbow & multicolor captions

Per word colour cycling caption styles, where each word takes the next colour in the sequence.

They render identically in the live preview and in the export, which is less obvious than it sounds: the preview draws with the browser's text engine and the export burns subtitles with a different one, so a per word colour had to be expressed in a way both understand.

Used sparingly they carry energy. Used on a two minute clip they are exhausting, and we would rather say that than sell them as a default.