Fix frac-line

[ronkok]

For frac-line, use an SVG line in an extra tall span.

I still need to think of a way to adust vertical-separator.

[ronkok]

Were the vertical separators disappearing at times? Won't this change to the styling affect the horizontal layout slightly?

Revisiting my response to this comment:

1. Yes. Issue Dividing lines for array environment sometimes disappear #970
2. No. It turns out that vertical-separator lines do nothing to the inter-column spacing of an {array}. In the screen shot below, I've drug part of a no-line array over and overlaid it onto an {array}{c|c}. The columns still align perfectly.

In the code for this PR, it was only necessary to adjust the vertical-separator negative left margin. No action is necessary to further fix the horizontal layout.

I believe that we're good to go.

[kevinbarabash]

LGTM

[kevinbarabash]

@ronkok you'll have to merge master into your branch and resolve the conflict. I can regenerate those screenshots afterwards.

[kevinbarabash]

@ronkok thank you for tackling this this issue with the fraction bars disappearing. I'm going to publish a release over the next couple of days. With this change I think it's safe to make it a beta release or release candidate. :)

[ronkok]

@kevinbarabash You're very welcome. I certainly think that we've now got something better than when we started. I hope it's enough.