webview/katex: Add hack so `\frac` lines don't disappear.
webview/katex: Add hack so `\frac` lines don't disappear.

KaTeX `.frac-line`s don't show up on some of our Android test devices
and emulators. (This seems to be a perennial problem for KaTeX. See
the commit-internal comments for references.)

Resolve this by forcing all such lines to be 1px wide, disregarding
the TeXbook spec.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile