[css-inline] Allow background of an inline area to extend to before-and after-edges of the line area #1974

Per the 20171109 joint TTWG/CSS meeting

As illustrated at fillLineGap.pdf, a common captioning style requires the background of inline areas to extend to the before-and after-edges of the containing line area. This styling capability is not currently supported by CSS.

This capability mirrors the itts:fillLineGap styling attribute at https://www.w3.org/TR/ttml-imsc1.0.1/#itts-fillLineGap