Mechanism to scale SVG images when default face height changes?

[Jimmy Yuen Ho Wong <wyuenho@gmail.com>]

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Hi everyone,

all-the-icons maintainer here. Recently I've converted all-the-icons from
using icon-fonts to SVGs. A few early testers of this branch have reported
that when they resize the default face, either by text-scale-mode, loading
a different theme or setting the default face size attributes manually, the
SVG icons inserted into the buffers do not scale accordingly.

In the SVG branch in all-the-icons, all the icons are inserted with a
`:width` and `:height` properties set on the image, and the face is applied
to the propertized string. i.e.

```elisp
ELISP> (all-the-icons-icon-for-file "test.js")
#("" 0 1
  (face all-the-icons-lpurple font-lock-face all-the-icons-lpurple
fontified t display
        (image :type svg :data "<svg viewBox=\"0 0 512 512\" width=\"512\"
height=\"512\"> <path d=\"...snip...\"></path></svg>" :scale 1
:transform-smoothing t :width 14 :height 14 :ascent center :margin 1)
        front-sticky nil rear-nonsticky t))
```

As far as I understand, simply adjusting the scale factor of the image will
suffice. The problem is, since Emacs doesn't do it by itself, I don't
really know how to do it in Elisp and be performant. Are there any hooks or
internal undocumented functions that I can use to scale all the images in
all of the frames when the default face size changes?

Thanks,

Jimmy Yuen Ho Wong

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