Add support for Region Bounding Volume

[gkjohnson]

Regions should be specified in the local frame of the group treating the group as the WGS84 coordinate frame.

Example data here.

Spec here.

More information in CesiumGS/3d-tiles#445

[gkjohnson]

Add support for:

• A Region class
• A RegionHelper class for rendering a region
• A displayBoundingRegion flag for debug tiles renderer

The region must be defined on the oblate WGS84 spheroid with major and minor axes defined here

[gkjohnson]

A simple implementation would just involve creating a sphere or box around the region volume.

[gkjohnson]

From #269 (comment):

A region bounding volume is defined here:

https://github.com/CesiumGS/3d-tiles/tree/main/specification#region

It's a patch defined by a latitude, longitude, and height delta on the WGS 84 ellipsoid. A simple approach to supporting a region volume is to generate a sphere from the region coordinates so that can be used for frustum culling etc. That could be added here:

3DTilesRendererJS/src/three/TilesRenderer.js

Lines 538 to 543 in 3f02bcb

	const region = null;
	if ( 'region' in tile.boundingVolume ) {
	console.warn( 'ThreeTilesRenderer: region bounding volume not supported.' );
	}