SVG conversion to PNG ignores <style> variables

Nobody has claimed this yet.

## Possible bug

SVG conversion to PNG ignores variables.

This issue limits compatibility with several of the assets I am using, and with 100% that I generate.

Maybe... this could be because of libvips, which uses librsvg. Acccording to this post, they still do not support style variables:
https://gitlab.gnome.org/GNOME/librsvg/-/issues/459

Even if the issue is because of a dependent package, it is important to track it until it is supported and be transparent to developers that use style variables.

### Are you using the latest version of sharp?

- [x] I am using the latest version of `sharp` as reported by `npm view sharp dist-tags.latest`.

### What is the output of running `npx envinfo --binaries --system --npmPackages=sharp --npmGlobalPackages=sharp`?

```
System:
OS: macOS 11.7
CPU: (4) x64 Intel(R) Core(TM) i5-7500 CPU @ 3.40GHz
Memory: 6.26 GB / 32.00 GB
Shell: 5.8 - /bin/zsh
Binaries:
Node: 16.14.0 - /usr/local/bin/node
npm: 8.15.1 - /usr/local/bin/npm
npmPackages:
sharp: ^0.31.2 => 0.31.2

```
### What are the steps to reproduce?

- Convert the attached sample SVG to PNG

### What is the expected behaviour?

- Output should have a blue background, as defined in the --Bg variable, but the background is black.

### Please provide a minimal, standalone code sample, without other dependencies, that demonstrates this problem

This is a nextjs api endpoint:

```
const sharp = require('sharp');
const sharpOptions = {
compressionLevel: 0,
quality: 100,
}

export default async function handler(request, response) {
const svg = '<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" version="1.1" width="600" height="600" viewBox="0 0 20 20"><defs><style>:root{--Bg:#0000FF;}.Bg{fill:var(--Bg);}.Rect{fill:#00ff00;}';

const svgBuffer = Buffer.from(svg);
const png = sharp(svgBuffer).png(sharpOptions);
const pngData = await png.toBuffer();

response.statusCode = 200;
response.setHeader('Content-Type', 'image/png');
response.setHeader('Cache-Control', 'public, max-age=0, must-revalidate');
response.end(pngData);
}
```

### Please provide sample image(s) that help explain this problem


Start by reproducing the conversion with the standalone sharp example and attached SVG, then check the referenced libvips/librsvg limitation at GNOME issue 459. Done means establishing whether the dependency now supports CSS style variables and documenting the resulting sharp behavior or limitation.

Written by the indexing model from the issue text.