Allow user to create background image from map services

I created a project that allows the user to create a "screenshot" of an _ArcGIS API for JavaScript_ `esri/Map` using HTML [Canvas API](https://developer.mozilla.org/en-US/docs/Web/API/Canvas_API). This would be a useful feature for your _ThumbnailBuilderUI_ for the background images section.

It **might** also be possible to use the [Canvas API](https://developer.mozilla.org/en-US/docs/Web/API/Canvas_API) to replace the call to the [GP service](https://github.com/sirws/ThumbnailBuilderServer) and do everything client-side.
- [Demo](http://wsdot-gis.github.io/arcgis-map-thumbnail-builder/)
- [Source](https://github.com/WSDOT-GIS/arcgis-map-thumbnail-builder/)