_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.