Bug: Latex preview overlay scaling not consistent across PPI

[Scott Otterson <scotto@sharpleaf.org>]

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The size of the latex preview overlay scaling is inconsistent across
screens with different pixel densities.

I've set my customization to use dvipng for the latex previews I see when I
type C-c C-x C-l.  Dvipng is customized as follows:

[image: Inline image 1]

The "Value" is set to (1.0 . 1.0), which I would think means that the font
size in the preview overlay is scaled the same as the font size org
buffer's normal text.

This is more or less true for a Windows 7 machine connected to a 24",
1920x1080 pixel monitor:

[image: Inline image 2]

However, on a Windows 10 machine with a 12.3", 2736x1824 pixel monitor, the
same customization looks like this:

[image: Inline image 3]

On the second computer (a Surface Pro 4) the previews are tiny because the
screen has a much higher pixel density; the normal text is scaled
appropriately but the overlays aren't scaled to match.

I'd like keep the same .emacs file across all the machines I have to use,
so it would be best if the preview scaling was consistent.

Possibly the overlay scaling calculation is done using the function
window-text-width without the *pixelwise *argument, which would return a
width in characters.  To avoid changing the calculation too much, you could
use window-max-chars-per-line, which considers font size and some things
that would be missed by window-text-width even if *pixelwise* was used.

-- my setup
------------------------------------------------------------------------------------------
Emacs  : GNU Emacs 25.1.1 (i686-w64-mingw32)
 of 2016-09-17
Package: Org mode version 9.0.1 (9.0.1-elpaplus @
c:/Users/sotterson/home/.emacs.d/elpa/org-plus-contrib-20161118/)

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