Beautiful Math Feature Request: Ability To Increase DPI

Related to: https://github.com/Automattic/jetpack/issues/2360

#### Steps to reproduce the issue

Looking at this code:
https://github.com/Automattic/jetpack/blob/master/modules/latex.php

And this doc:
https://jetpack.com/support/beautiful-math-with-latex/

I see that I can render in "Huge" mode by setting `s=4`.

#### What I expected

A high quality image suitable for printing in a book.

#### What happened instead

Unfortunately, even at this setting, the image is is not high quality enough for print.

#### Feature request

Assuming the underlying toolset behind `wp.com/latex.php` is the same as [this plugin](https://github.com/wp-plugins/wp-latex/blob/master/automattic-latex-dvipng.php#L255), would it be possible to expose the `-D` flag for `AUTOMATTIC_LATEX_DVIPNG_PATH` up to a reasonable maximum? Like, I could call your API with `d=300` or something?

Thank you for your consideration.

Read modules/latex.php and compare the linked wp-latex implementation, especially automattic-latex-dvipng.php. Determine where rendering options are handled and how an API value such as d should map to the -D flag. Done means a bounded DPI value can request higher-quality output, with the Beautiful Math documentation updated to describe the option.

Written by the indexing model from the issue text.