Add background transparent if not set in SMPTE-TT subtitles

[avelad]

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[shaka-bot]

All tests passed!

[ismena]

@joeyparrish had concerns about this approach.
Joey, can you take over the conversation?

@avelad if you don't hear from Joey in two days, please ping me here, I'll review using my best judgement :)

[avelad]

The commit 6aa35d1#diff-05a39ed9405ffc286c9f33f36ada332b put a black background by default. But the images should be a transparent background by default (it's possible override the behaviour in the ttml style)

[joeyparrish]

Since your change to the default is inside of if (cue.backgroundImage), I think this is fine. It's reasonable to have a transparent default when there's a background image.