GetResource Images - back fill missing images

[dasgarner]

Any images that don't load should be replaced by a 1px transparent square, a background retry should be attempted to back fill these images if they become available.

[dasgarner]

There is already something at the end of xibo-text-render.js which attempts to replace any error'ed images with a transparent PNG.

Perhaps should could be separated into its own JS file (and included in each relevant module), but also enhanced to periodically re-request the file and insert it if present.