Multiple state variables refer to same thing in webvtt_cue and WebVTT::Cue

`line_is_relative` and `snap_to_lines` are two separate state variables that refer to the same thing.

They are both taken from the spec, and refer to the same thing. But we're not actually maintaining both of them. So, we should remove one.

I think snapToLines looks cleaner, so I vote we use that.

```C++
inline bool snapToLines() const { return !!cue->snap_to_lines; }

...
...
inline bool isRelativeLinePosition() const { return !!(cue->settings.line_is_relative); }
```