Canvas shouldn't force to embed the image in the html

[rmannibucau]

asciidoctor-bespoke/templates/slim/section.html.slim

Line 11 in d402d63

- _style = %[background-image: url(#{image_uri _bg_img.attr(:target)}); background-size: #{(_bg_img.roles & %w[cover contain])[0] || 'cover'}; background-repeat: no-repeat]

use background-url with a data uri but this lead to big html files.
On some http servers it can't be served (*.html max size <= 1M). For 5 slides it overpasses quickly 1M!

Would be great to be able to use an actual url.