WebGLShadowMap: Make sure USE_SHADOWMAP is defined for distanceMaterial. Fixes #8166.

src/renderers/webgl/WebGLShadowMap.js

@@ -64,6 +64,9 @@ THREE.WebGLShadowMap = function ( _renderer, _lights, _objects ) {
 		_depthMaterials[ i ] = depthMaterial;
 
 		var distanceMaterial = new THREE.ShaderMaterial( {
+			defines: {
+				'USE_SHADOWMAP': ''
+			},
 			uniforms: distanceUniforms,
 			vertexShader: distanceShader.vertexShader,
 			fragmentShader: distanceShader.fragmentShader,