Draw shapes with signed distance fields

[olivervogel]

libvips 8.16 has a new way to draw shapes with signed distance fields. Currently SVG is used to draw all shapes onto images.

https://www.libvips.org/2024/10/10/What's-new-in-8.16.html

Since the new method is faster and need less memory this could be an improvement. We may wait to implement it until libvips 8.16 is more widely available/used.