So I need to create a function wrapping a variadic c function (https://jcupitt.github.io/libvips/API/current/libvips-resample.html#vips-thumbnail) in rust.
This function takes some arguments and lastly a "NULL-terminated list of optional named arguments".
c function:
int vips_thumbnail (const char *filename, VipsImage **out, int width, ...);
example calling it from rust with a optional height parameter:
vips_thumbnail(some_path.as_ptr(), &mut out_ptr, width as i32, "height\0".as_ptr(), height as i32, null() as *const c_char);