Support WebGLRenderTargetCube background

src/renderers/webgl/WebGLBackground.js

@@ -40,7 +40,7 @@ function WebGLBackground( renderer, state, objects, premultipliedAlpha ) {
 
 		}
 
-		if ( background && background.isCubeTexture ) {
+		if ( background && ( background.isCubeTexture || background.isWebGLRenderTargetCube ) ) {
 
 			if ( boxMesh === undefined ) {
 
@@ -70,7 +70,8 @@ function WebGLBackground( renderer, state, objects, premultipliedAlpha ) {
 
 			}
 
-			boxMesh.material.uniforms.tCube.value = background;
+			boxMesh.material.uniforms.tCube.value = ( background.isWebGLRenderTargetCube ) ? background.texture : background;
+			boxMesh.material.uniforms.tFlip.value = ( background.isWebGLRenderTargetCube ) ? 1 : - 1;
 
 			renderList.push( boxMesh, boxMesh.geometry, boxMesh.material, 0, null );