_blur( cubeUVRenderTarget, lodIn, lodOut, sigma, poleAxis ) {

		const pingPongRenderTarget = this._pingPongRenderTarget;

		this._halfBlur(
			cubeUVRenderTarget,
			pingPongRenderTarget,
			lodIn,
			lodOut,
			sigma,
			'latitudinal',
			poleAxis );

		this._halfBlur(
			pingPongRenderTarget,
			cubeUVRenderTarget,
			lodOut,
			lodOut,
			sigma,
			'longitudinal',
			poleAxis );

	}
This is a two-pass Gaussian blur for a cubemap. Normally this is done
vertically and horizontally, but this breaks down on a cube. Here we apply
the blur latitudinally (around the poles), and then longitudinally (towards
the poles) to approximate the orthogonally-separable blur. It is least
accurate at the poles, but still does a decent job.

@private
@param {WebGLRenderTarget} cubeUVRenderTarget
@param {number} lodIn
@param {number} lodOut
@param {number} sigma
@param {Vector3} [poleAxis]