_initializeBlurPlane( res, lightMap = null ) {

		const blurMaterial = new MeshBasicMaterial();
		blurMaterial.uniforms = { previousShadowMap: { value: null },
								  pixelOffset: { value: 1.0 / res },
								  polygonOffset: true, polygonOffsetFactor: - 1, polygonOffsetUnits: 3.0 };
		blurMaterial.onBeforeCompile = ( shader ) => {

			// Vertex Shader: Set Vertex Positions to the Unwrapped UV Positions
			shader.vertexShader =
				'#define USE_UV\n' +
				shader.vertexShader.slice( 0, - 1 ) +
				'	gl_Position = vec4((uv - 0.5) * 2.0, 1.0, 1.0); }';

			// Fragment Shader: Set Pixels to 9-tap box blur the current frame's Shadows
			const bodyStart	= shader.fragmentShader.indexOf( 'void main() {' );
			shader.fragmentShader =
				'#define USE_UV\n' +
				shader.fragmentShader.slice( 0, bodyStart ) +
				'	uniform sampler2D previousShadowMap;\n	uniform float pixelOffset;\n' +
				shader.fragmentShader.slice( bodyStart - 1, - 1 ) +
					`	gl_FragColor.rgb = (
									texture2D(previousShadowMap, vUv + vec2( pixelOffset,  0.0        )).rgb +
									texture2D(previousShadowMap, vUv + vec2( 0.0        ,  pixelOffset)).rgb +
									texture2D(previousShadowMap, vUv + vec2( 0.0        , -pixelOffset)).rgb +
									texture2D(previousShadowMap, vUv + vec2(-pixelOffset,  0.0        )).rgb +
									texture2D(previousShadowMap, vUv + vec2( pixelOffset,  pixelOffset)).rgb +
									texture2D(previousShadowMap, vUv + vec2(-pixelOffset,  pixelOffset)).rgb +
									texture2D(previousShadowMap, vUv + vec2( pixelOffset, -pixelOffset)).rgb +
									texture2D(previousShadowMap, vUv + vec2(-pixelOffset, -pixelOffset)).rgb)/8.0;
				}`;

			// Set the LightMap Accumulation Buffer
			shader.uniforms.previousShadowMap = { value: lightMap.texture };
			shader.uniforms.pixelOffset = { value: 0.5 / res };
			blurMaterial.uniforms = shader.uniforms;

			// Set the new Shader to this
			blurMaterial.userData.shader = shader;

		};

		this.blurringPlane = new Mesh( new PlaneGeometry( 1, 1 ), blurMaterial );
		this.blurringPlane.name = 'Blurring Plane';
		this.blurringPlane.frustumCulled = false;
		this.blurringPlane.renderOrder = 0;
		this.blurringPlane.material.depthWrite = false;
		this.scene.add( this.blurringPlane );

	}
Creates the Blurring Plane.

@private
@param {number} res - The square resolution of this object's lightMap.
@param {WebGLRenderTarget} [lightMap] - The lightmap to initialize the plane with.