Define markup for orthophoto image support

[prushforth]

Per UCR Issue, we should design and specify the markup to encode orthophoto georeferencing.

[prushforth]

Leaflet has (very) basic support for image overlays, but there are several overlay plugins that might give us ideas on what simple georeferencing markup might be.

Unsure about OpenLayers, more research required.

There is also the georeferencing information that is typically associated to imagery via embedded metadata e.g. in geotiff files, which sometimes have a ".world" file that associates the metadata externally. We should review that stuff to see if something simple can be found.