It's the only tutorial where we used the extension for *3* formulas which anyway
ended up as fairly low res static images generated by sphinx.ext.imgmath, so
requiring ~1 GB of LaTeX setup for it is overkill.
The replacement pictures are the ones generated by sphinx.ext.imgmath, we could
replace them by higher resolution ones.
Improve CI build times by using a dummy builder for (empty) output, dropping LaTeX, and disabling some Sphinx extensions when building for CI (GDScript lexing, creating a search index, and generating HTML description tags).
In my tests, this improves Travis CI build times: from up to 30 minutes down to 2 minutes.