Add test for fillLineGap when background is semi-transparent

[nigelmegitt]

When the background colour is semi-transparent, any overlapping backgrounds become visible. Add a test, e.g. a variant of fillLineGap001 with transparency set to 50%.

[css-meeting-bot]

The Timed Text Working Group just discussed Add test for fillLineGap when background is semi-transparent w3c/imsc-tests#99, and agreed to the following:

• SUMMARY: Group asked to review pull request

The full IRC log of that discussion

<nigel> Topic: Add test for fillLineGap when background is semi-transparent #99
<nigel> github: https://github.com//issues/99
<nigel> Nigel: Just to alert people to this, and mention the motivation.
<nigel> .. I was experimenting with varying the opacity of tts:backgroundColor on span and found that when fillLineGap is enabled,
<nigel> .. there's a high visual sensitivity to exact boundaries of spans, but no test cases, so I opened the pull request for this issue to add one.
<nigel> .. May also be somewhat relevant for linePadding.
<nigel> .. Please take a look.
<nigel> SUMMARY: Group asked to review pull request