Background Image: apply markup once loaded

We were getting consistent errors during server-side markup rendering
saying:

 Error: Image given has not completed loading
     at klass.fabric.Image.fabric.util.createClass._render (/fabric.js/dist/all.js:10560:11)
     at klass.fabric.Image.fabric.util.createClass.render (/fabric.js/dist/all.js:10398:12)
     at extend._draw (/fabric.js/dist/all.js:3497:16)
     at extend.renderAll (/fabric.js/dist/all.js:3613:16)
     at writeCanvas (/node-markup/ImageMarkupBuilder.js:290:20)
     at applyMarkup (/node-markup/ImageMarkupBuilder.js:280:10)
     at /node_programs/node-markup/ImageMarkupBuilder.js:227:19
     at fabric.Image.fromObject (/fabric.js/dist/all.js:10835:7)
     at Object.fabric.util.loadImage (/fabric.js/dist/all.js:10791:28)
     at Function.fabric.Image.fromObject (/fabric.js/dist/all.js:10830:17)