Hello,

I've created a patch to align the baseline of latex image fragments to
the surrounding text. The patch consists of several parts. First, it
adds a customizable variable that can be set to a user supplied function
to compute the value of :ascent passed to `overlay-put'. It can also be
set to a symbol (e.g., 'center, which is the current setting and still
the default), or an integer (:ascent can take an integer
percentage). The patch also modifies `org--make-preview-overlay' to use
this new variable.

The other part of the patch is a new function that computes the correct
value for :ascent from an SVG file.

Unfortunately, this isn't a general-purpose solution to baseline text
alignment. It currently only works with SVG images and requires that
these SVG images encode the text baseline in the viewBox attribute (I've
explained in the function documentation how to achieve this).

I was not initially planning to include the function because it only
works in certain special cases (though if you setup your SVG image
generation as I've described in the documentation it should work
correctly for all of your LaTeX fragments). However, I ultimately
decided it was beneficial to include it for several reasons: (1) it
would not be obvious for many users how to write this function on their
own, (2) by including it, others can improve upon it, and (3) I tried to
minimize corner cases by leaving the default value as 'center and having
the function return 'center whenever it detects an incorrect :ascent
value.

Let me know what you think!

Best
Matt