WebGLShadowMap: Use state.clearColor().

src/renderers/webgl/WebGLShadowMap.js

@@ -97,7 +97,7 @@ THREE.WebGLShadowMap = function ( _renderer, _lights, _objects ) {
 		if ( scope.autoUpdate === false && scope.needsUpdate === false ) return;
 
 		// Set GL state for depth map.
-		_gl.clearColor( 1, 1, 1, 1 );
+		_state.clearColor( 1, 1, 1, 1 );
 		_state.disable( _gl.BLEND );
 		_state.enable( _gl.CULL_FACE );
 		_gl.frontFace( _gl.CCW );