Embed does not expose the background colour as an option

[NickBlow]

https://www.libvips.org/API/current/libvips-conversion.html#vips-embed exposes a background colour as an option, but you can't pass it into GoVips. This means that the VIPS_EXTEND_BACKGROUND enum also does nothing in this case.

[tonimelisma]

I'll mark it as a feature request. PRs are always welcome and I'll try to speedily review them