deriveChildTile(
		subtree,
		parentTile,
		childBitIndex,
		childMortonIndex
	) {

		const subtreeTile = new SubtreeTile( parentTile, childMortonIndex );
		subtreeTile.boundingVolume = this.getTileBoundingVolume( subtreeTile );
		subtreeTile.geometricError = this.getGeometricError( subtreeTile );
		// Todo Multiple contents not handled, keep the first found content.
		for ( let i = 0; subtree && i < subtree._contentAvailabilityBitstreams.length; i ++ ) {

			if ( subtree && this.getBit( subtree._contentAvailabilityBitstreams[ i ], childBitIndex ) ) {

				subtreeTile.content = { uri: this.parseImplicitURI( subtreeTile, this.rootTile.content.uri ) };
				break;

			}

		}
		return subtreeTile;

	}
Given a parent tile and information about which child to create, derive
the properties of the child tile implicitly.
<p>
This creates a real tile for rendering.
</p>

@param {Subtree} subtree The subtree the child tile belongs to.
@param {Object | SubtreeTile} parentTile The parent of the new child tile.
@param {number} childBitIndex The index of the child tile within the tile's availability information.
@param {number} childMortonIndex The morton index of the child tile relative to its parent.
@returns {SubtreeTile} The new child tile.
@private