dirname() is a slow function compared with __DIR__ and using
'/../'. Moodle has a large number of legacy files that are included
each time a page loads and is not able to use an autoloader as it is
functional code. This allows those required includes to perform as
best as possible in this situation.
This equation editor relies on whatever the currently configured filter is to do the rendering.
If there is no filter that handles latex ($$ blah $$) - this plugin will not show up.
This will not work with the solutions on the forums of adding MathJax in the header of the page,
but it will work with a real mathjax filter written for Moodle (work in progress). It works with
the existing tex filter.