Implemented @alteredq's PointLight Shadow version using 4 taps instead of 21. See #7153.

https://twitter.com/alteredq/status/697103627773734914

src/renderers/shaders/ShaderChunk/shadowmap_pars_fragment.glsl

@@ -213,31 +213,14 @@
 
 		#if defined( SHADOWMAP_TYPE_PCF ) || defined( SHADOWMAP_TYPE_PCF_SOFT )
 
-			vec3 offset = vec3( - 1, 0, 1 ) * shadowRadius * 2.0 * texelSize.y;
+			vec2 offset = vec2( - 1, 1 ) * shadowRadius * 2.0 * texelSize.y;
 
 			return (
-				texture2DCompare( shadowMap, cubeToUV( bd3D + offset.zzz, texelSize.y ), dp ) +
-				texture2DCompare( shadowMap, cubeToUV( bd3D + offset.zxz, texelSize.y ), dp ) +
-				texture2DCompare( shadowMap, cubeToUV( bd3D + offset.xxz, texelSize.y ), dp ) +
-				texture2DCompare( shadowMap, cubeToUV( bd3D + offset.xzz, texelSize.y ), dp ) +
-				texture2DCompare( shadowMap, cubeToUV( bd3D + offset.zzx, texelSize.y ), dp ) +
-				texture2DCompare( shadowMap, cubeToUV( bd3D + offset.zxx, texelSize.y ), dp ) +
-				texture2DCompare( shadowMap, cubeToUV( bd3D + offset.xxx, texelSize.y ), dp ) +
-				texture2DCompare( shadowMap, cubeToUV( bd3D + offset.xzx, texelSize.y ), dp ) +
-				texture2DCompare( shadowMap, cubeToUV( bd3D + offset.zzy, texelSize.y ), dp ) +
-				texture2DCompare( shadowMap, cubeToUV( bd3D + offset.zxy, texelSize.y ), dp ) +
-				texture2DCompare( shadowMap, cubeToUV( bd3D, texelSize.y ), dp ) +
-				texture2DCompare( shadowMap, cubeToUV( bd3D + offset.xxy, texelSize.y ), dp ) +
-				texture2DCompare( shadowMap, cubeToUV( bd3D + offset.xzy, texelSize.y ), dp ) +
-				texture2DCompare( shadowMap, cubeToUV( bd3D + offset.zyz, texelSize.y ), dp ) +
-				texture2DCompare( shadowMap, cubeToUV( bd3D + offset.xyz, texelSize.y ), dp ) +
-				texture2DCompare( shadowMap, cubeToUV( bd3D + offset.zyx, texelSize.y ), dp ) +
 				texture2DCompare( shadowMap, cubeToUV( bd3D + offset.xyx, texelSize.y ), dp ) +
-				texture2DCompare( shadowMap, cubeToUV( bd3D + offset.yzz, texelSize.y ), dp ) +
-				texture2DCompare( shadowMap, cubeToUV( bd3D + offset.yxz, texelSize.y ), dp ) +
 				texture2DCompare( shadowMap, cubeToUV( bd3D + offset.yxx, texelSize.y ), dp ) +
-				texture2DCompare( shadowMap, cubeToUV( bd3D + offset.yzx, texelSize.y ), dp )
-			) * ( 1.0 / 21.0 );
+				texture2DCompare( shadowMap, cubeToUV( bd3D + offset.xxx, texelSize.y ), dp ) +
+				texture2DCompare( shadowMap, cubeToUV( bd3D + offset.yxy, texelSize.y ), dp )
+			) * ( 1.0 / 4.0 );
 
 		#else // no percentage-closer filtering