addGroup( start, count, materialIndex = 0 ) {

		this.groups.push( {

			start: start,
			count: count,
			materialIndex: materialIndex

		} );

	}
Adds a group to this geometry.

@param {number} start - The first element in this draw call. That is the first
vertex for non-indexed geometry, otherwise the first triangle index.
@param {number} count - Specifies how many vertices (or indices) are part of this group.
@param {number} [materialIndex=0] - The material array index to use.