Since all the math font test cases use the KaTeX logo, we need that.
I started with the definition of the logo from katex.less, but tweaked that
until it gave a good visual match, in particular a very similar logo width,
no matter the actual numbers.
With that logo, most tests can be compiled again, with the exception of the
one containing illegal functions to test visual error reporting.
That one needs to be explicitely disabled.
The same test cases we use for our screenshots from Firefox are now also
being rendered by pdflatex, so the resulting images can be used as reference
for how things are supposed to look (if we concentrate on compatibility with
LaTeX). To make comparisons even easier, the differences between LaTeX and
Firefox snapshots are rendered in a visual way, using different colors.
Discussed in pull request #268.
