backward compatibility

src/renderers/WebGLRenderer.js

@@ -1089,7 +1089,7 @@ function WebGLRenderer( parameters ) {
 		currentRenderState = renderStates.get( scene, camera );
 		currentRenderState.init();
 
-		scene.onBeforeRender( _this, scene, camera, _currentRenderTarget );
+		scene.onBeforeRender( _this, scene, camera, renderTarget || _currentRenderTarget );
 
 		_projScreenMatrix.multiplyMatrices( camera.projectionMatrix, camera.matrixWorldInverse );
 		_frustum.setFromMatrix( _projScreenMatrix );