Enable all layer for WebGLBackgrond.

Before mrdoob#22426,
when not using array camera, the background doesn't participate
in layer calculation.

We feel the old behaviour is better because we don't need to
always add layer `0` to the camera when doing multipass rendering.

This commit enables all layer for the background.

Note that the behaviour is still slightly different from r115:

When the camera's layer **mask** is set to 0, r115 will render
background and now won't.

Author: chubei-oppen

src/renderers/webgl/WebGLBackground.js

@@ -98,6 +98,7 @@ function WebGLBackground( renderer, cubemaps, state, objects, alpha, premultipli
 						fog: false
 					} )
 				);
+				boxMesh.layers.enableAll();
 
 				boxMesh.geometry.deleteAttribute( 'normal' );
 				boxMesh.geometry.deleteAttribute( 'uv' );
@@ -161,6 +162,7 @@ function WebGLBackground( renderer, cubemaps, state, objects, alpha, premultipli
 						fog: false
 					} )
 				);
+				planeMesh.layers.enableAll();
 
 				planeMesh.geometry.deleteAttribute( 'normal' );