deprecated renderTarget and forceClear from WebGLRenderer.render()

resolves #15420

src/renderers/WebGLRenderer.js

@@ -1024,7 +1024,24 @@ function WebGLRenderer( parameters ) {
 
 	// Rendering
 
-	this.render = function ( scene, camera, renderTarget, forceClear ) {
+	this.render = function ( scene, camera ) {
+
+		var renderTarget;
+		var forceClear;
+
+		if ( arguments[ 2 ] !== undefined ) {
+
+			console.warn( 'THREE.WebGLRenderer.render(): the renderTarget argument has been removed. Use .setRenderTarget() instead.' );
+			renderTarget = arguments[ 2 ];
+
+		}
+
+		if ( arguments[ 3 ] !== undefined ) {
+
+			console.warn( 'THREE.WebGLRenderer.render(): the forceClear argument has been removed. Use .clear() instead.' );
+			forceClear = arguments[ 3 ];
+
+		}
 
 		if ( ! ( camera && camera.isCamera ) ) {
 
@@ -1097,14 +1114,12 @@ function WebGLRenderer( parameters ) {
 
 		if ( this.info.autoReset ) this.info.reset();
 
-		if ( renderTarget === undefined ) {
+		if ( renderTarget !== undefined ) {
 
-			renderTarget = null;
+			this.setRenderTarget( renderTarget );
 
 		}
 
-		this.setRenderTarget( renderTarget );
-
 		//
 
 		background.render( currentRenderList, scene, camera, forceClear );