Apply a padding overlay box with a margin of 1.5%.

I've also started to round the font size setting to
simplify the JSON files a bit.
Turn processing model tests that were accidentally turned off back on.

I've added a new test-runner method that tests both parsing, without
streaming, and processing whole VTT files.
Wrap the cue in an anonymous div with inline positioning.

This functions as the cue background box and allows us to only
have the text have the background color. As well as allowing us
to use getClientRects() to get each line of text as a TextRectangle
which will allow us to get the exact line heights.
Work.