Plugins: Add "Columbus View" plugin for ellipsoidal data sets

Provide a plugin to transform an ellipsoid data set into a flat, equirect one. Could be used for relief-style maps with Google tiles, qmesh, lunar, etc

In Cesium this is referred to as a "Columbus" view. A generalized method for "unwrapping" geometry known to be ellipsoid-ally aligned would enable Google Photorealistic Tiles, 2d map tiles, quantized mesh, etc to be more easily unwrapped into a consistent format. A subsequent step would be enabling geospatial-coordinate-aware plugins (such as overlays) to overlay or position data on the 2d map position.

Some things to be figured out:

• What should the plugin be called?
  • Relevant words: "2.5D", "Relief", "Unwrapped", "Columbus View"
• What cartesian range should the data be mapped to?
• Should the plugin provide a function for resolving cartographic coordinates to cartesian?
• How can the other geospatial-aware plugins know how the map their data to the 2d space? Right now they invert the lat / lon / height from the ellipsoid.
• How to handle "clipped" / "split" geometry (eg from image overly plugin and quantized mesh)? The plugin will have already "unwrapped" it? Possibly related to #857?
  • #1578 could help provide the above 3 (possibly 4) points

Related to #1347

Start by reviewing related issues #1347, #1578, and #857, then inspect the existing plugin approach for geospatial data. The scope requires resolving the naming, coordinate mapping, cartographic conversion, and clipped or split geometry questions before implementation. Done means an agreed Columbus View plugin can consistently unwrap ellipsoidal data into a flat equirectangular format.