Add a "SmoothTerrainPlugin" for generating flat maps

[gkjohnson]

Related to #1473
Related to #1347

This will be used for generating smooth ellipsoid terrain for overlays to be projected on to. Then the "EPSG" / Ellipsoid Image plugins can be deprecated. This could also provide better handling / mapping for "flat" maps so lat / lon can be queried & located more easily.

Other name options:

• "MapPlugin"
• "BaseTerrainPlugin"
• "TilesGridPlugin"
• "FlatTilesPlugin"
• "GlobeTilesPlugin"

Considerations:

• We should need to add a "fast path" to ImageOverlayPlugin so that it just uses the individual texture tile directly rather than composing a render target if the bounds of the geometry tile match exactly in order to save memory. (fixed in ImageOverlayPlugin: Use the original texture directly if the bounds match geometry exactly. #1535)
• We need some way of allow the underlying geometry to be derived from a common overlay as those used in ImageOverlayPlugin.
  • Allow for ImageOverlayPlugin to generate geometry? Or create the above plugin?
  • Pass a reference of the geometry plugin to the ImageOverlayPlugin?
  • Pass the overlay (or image source?) to the TilesGridPlugin. Otherwise generate based on a default schema (optionally passing a ESPG frame)
• Should the base geometry plugin be split into "Globe" and "Flat" varients?
• Plugins like "deep zoom" etc all need an overlay definition and it we need to figure out how it works (or doesn't) with globe projections

To Start

• Handle globe tiles, first

[gkjohnson]

const xyzOverlay = new XYZTilesOverlay();
const overlayPlugin = new ImageOverlayPlugin( {
  overlay: [ xyzOverlay ],
} );
const globePlugin = new GlobeTilesPlugin( {
  scheme: xyzOverlay,
  // or
  scheme: 'EPSG:4326',
} );

tiles.registerPlugin( overlayPlugin );
tiles.registerPlugin( globePlugin );