Prepend the base path to the remote file path when registering font.

Useful when the `$remote_file` is a relative URL like `/assets/fonts/glyphicons-halflings-regular.woff`:
```css
@font-face {
  font-family: 'Glyphicons Halflings';
  src: url(/assets/fonts/glyphicons-halflings-regular.woff);
}
```
Avoid error like `file_get_contents(/assets/fonts/glyphicons-halflings-regular.woff): failed to open stream: No such file or directory` in `vendor/dompdf/dompdf/include/font_metrics.cls.php`.