极坐标格辅助对象. 坐标格实际上是2维线数组.
		const radius = 10;
		const sectors = 16;
		const rings = 8;
		const divisions = 64;
		const helper = new THREE.PolarGridHelper( radius, sectors, rings, divisions );
		scene.add( helper );
		
		[example:webgl_helpers WebGL / helpers]
		radius -- 极坐标格半径. 可以为任何正数. 默认为 10.
		sectors -- The number of sectors the grid will be divided into. This can be any positive integer. Default is 16.
		rings -- The number of rings. This can be any positive integer. Default is 8.
		divisions -- 圆圈细分段数. 可以为任何大于或等于3的正整数. 默认为 64.
		color1 -- 极坐标格使用的第一个颜色. 值可以为 [page:Color] 类型, 16进制 和 CSS 颜色名. 默认为 0x444444 
		color2 -- 极坐标格使用的第二个颜色. 值可以为 [page:Color] 类型, 16进制 和 CSS 颜色名. 默认为 0x888888
		
Creates a new [name] of radius 'radius' with 'sectors' number of sectors and 'rings' number of rings, where each circle is smoothed into 'divisions' number of line segments. Colors are optional.
[link:https://github.com/mrdoob/three.js/blob/master/src/[path].js src/[path].js]