Improve CEA-608 column to VTT position

[robwalch]

This PR will...

Calculate cue position for CEA-608 caption cues according to the PAC indent code table at https://dvcs.w3.org/hg/text-tracks/raw-file/default/608toVTT/608toVTT.html#positioning-in-cea-608

Why is this Pull Request needed?

Improve rendering of 608 captions (multiline and left aligned).

Are there any points in the code the reviewer needs to double check?

Also a small optimization on whitespace check using RegEx.test rather than String.match.

Test streams:
https://playertest.longtailvideo.com/adaptive/captions/playlist.m3u8
https://playertest.longtailvideo.com/adaptive/cpcweb/cpcdemo.m3u8

Checklist

• changes have been done against master branch, and PR does not conflict
• new unit / functional tests have been added (whenever applicable)
• API or design changes are documented in API.md

[robwalch]

@OrenMe some snippets that can help with testing:

1. "indent" to position output to compare to the PAC table in https://dvcs.w3.org/hg/text-tracks/raw-file/default/608toVTT/608toVTT.html#positioning-in-cea-608 (hopefully what we refer to as indent in our code matches the the spec):

for (let i = 0; i < 29; i++) {
    console.log(i, Math.max(10, 10 + Math.min(80, Math.floor(i * 8 / 32) * 10)));
}

2. This is more for visual testing of cue lines in native TextTrack rendering across browsers if you don't have a stream chock full of lines:

(function() {
  const video = document.querySelector('video');
  const track = video.addTextTrack('subtitles', 'English', 'en');
  track.mode = 'showing';
  const start = 1;
  const end = video.duration - 1;
  for (let i=15; i>12; i--) {
      const cue = new VTTCue(start, end, `Line ${i}`);
      cue.line = i;
      track.addCue(cue);
  }
}());

[OrenMe]

I actually ran the code for #1 when reviewing it and I saw it is with line of what we saw in the spec.
#2 is great for testing and I saw it also in the chromium bug report - thanks!

I'm checking about fixing the long tail to the right which we saw.