Fix color support for stretchy, strikethrough, and fbox

[xsznix]

Summary:
Stuff like \red{\overbrace{AB}} works now in addition to \color{red}{\overbrace{AB}}. Strikethrough now respects color. The Firefox in the screenshotter doesn't seem to support background-image + mask, but I manually tested that the latest Firefox does.

Test plan:
Ran make, then tested in latest Chrome and Firefox to ensure color support was working, then ran make screenshots.

[khanbot]

CLA signature looks good 👍

[ronkok]

Interesting. I did not realize that KaTeX supported that syntax. It does, though. \red{A} renders a red A.

Are there any LaTeX packages that support this syntax?

[edemaine]

Nor did I! Here's the code. And I guess there are matching katex-... colors in the CSS. I'm not aware of a LaTeX package implementing this -- probably a KA thing. IMO, these might make more sense as macros...

[ronkok]

Ah, I see. Well, @xymostech has written issue #28, which states that these custom colors were meant to be KA-only and not available for everyone. There is some push back in the discussion.

I suppose one approach would be to accept this PR, so that KaTeX has consistent behavior. Then, if a PR is ever written to resolve #28, it would have to address the whole issue.

[sophiebits]

Yes, the intent was always to move those to macros as soon as was possible.

I think this change seems pretty unambiguously good especially in the existing presence of the options.getColor method although I don't understand the significance of the CSS change.

[ronkok]

The CSS change fixes a bug which has crept into function stretchy.encloseSpan(). At one time, I pushed mask onto the list of classes for the relevant span. Somehow, I let that item slip out. The CSS in this PR corrects for my error.

Thank you, @xsznix!

Also, Firefox 53 was the first Firefox release with full mask support. I suppose Screenshotter is using an older version.

[edemaine]

In this case, perhaps we should accept this PR and then make a new one to rewrite \red etc. in terms of macros? I could use some clarification on whether those macros belong in KaTeX (e.g. for backwards compatibility) or in another KA project.

[edemaine]

LGTM. Thanks for the fixes, @xsznix!

[kevinbarabash]

In this case, perhaps we should accept this PR and then make a new one to rewrite \red etc. in terms of macros?

I think that makes sense.

I could use some clarification on whether those macros belong in KaTeX (e.g. for backwards compatibility) or in another KA project.

As per the comments in #28, we definitely want to move KA custom colors out of KaTeX but in a thoughtful way.

[kevinbarabash]

@xsznix thanks for the PR. @edemaine thanks for reviewing.