Add support for relative assets in `magick-composite`.
Add support for relative assets in `magick-composite`.

This makes possible to use `url(..)` or `image-url(..)` to pass in the path to the image, but only when `relative_assets = true`.
It shouldn't be too widely used, but may be required if `magick-composite` is chained with `magick-image` which generated a file, rather than a Base64 data-url.

StanAngeloff/compass-magick,StanAngeloff/compass-magick