SVG images not supported as background via CSS

In our project, we are using `PdfRendererBuilder.withW3cDocument` to convert a HTML document to PDF. In this input, we use CSS to define a background image:
```
.logo {
background-image: url("logo.svg");
}
```

When the PDF is rendered, we get the following stacktrace:

```
Can't read image file; unexpected problem for URI '.../logo.svg'

java.io.IOException: Unrecognized Image format
at com.openhtmltopdf.pdfboxout.PdfBoxImage.(PdfBoxImage.java:44)
at com.openhtmltopdf.pdfboxout.PdfBoxUserAgent.getImageResource(PdfBoxUserAgent.java:90)
at com.openhtmltopdf.render.AbstractOutputDevice.getBackgroundImage(AbstractOutputDevice.java:210)
at com.openhtmltopdf.render.AbstractOutputDevice.paintBackground0(AbstractOutputDevice.java:295)
at com.openhtmltopdf.render.AbstractOutputDevice.paintBackground(AbstractOutputDevice.java:234)
at com.openhtmltopdf.pdfboxout.PdfBoxFastOutputDevice.paintBackground(PdfBoxFastOutputDevice.java:283)
at com.openhtmltopdf.render.Box.paintBackground(Box.java:583)
at com.openhtmltopdf.render.displaylist.DisplayListPainter.paintBackgroundAndBorders(DisplayListPainter.java:75)
at com.openhtmltopdf.render.displaylist.DisplayListPainter.paint(DisplayListPainter.java:251)
at com.openhtmltopdf.pdfboxout.PdfBoxRenderer.paintPageFast(PdfBoxRenderer.java:921)
at com.openhtmltopdf.pdfboxout.PdfBoxRenderer.writePDFFast(PdfBoxRenderer.java:617)
at com.openhtmltopdf.pdfboxout.PdfBoxRenderer.createPdfFast(PdfBoxRenderer.java:554)
at com.openhtmltopdf.pdfboxout.PdfBoxRenderer.createPDF(PdfBoxRenderer.java:480)
at com.openhtmltopdf.pdfboxout.PdfBoxRenderer.createPDF(PdfBoxRenderer.java:417)
at com.openhtmltopdf.pdfboxout.PdfBoxRenderer.createPDF(PdfBoxRenderer.java:399)
at com.openhtmltopdf.pdfboxout.PdfRendererBuilder.run(PdfRendererBuilder.java:46)
[...]
```

When we add the SVG directly to the HTML (with ``), the image is rendered correctly. To make this work, we add the `com.openhtmltopdf.svgsupport.BatikSVGDrawer` in`PdfRendererBuilder.useSVGDrawer`.

The cause seems to be, that the SVG drawer is not used when rendering the background.

Thanks!