Make it possible to disable skybox rendering

It is currently not possible to set the prefiltered lighting data on the scene without also having the skybox render on the background.

You can achieve this by disabling the layer:

this.camera.camera.layers = this.camera.camera.layers.filter((l) => {
    return l !== pc.LAYERID_SKYBOX;
});

However this will still cause meshes and shaders to be created for the skybox unnecessarily.

Add an option to the scene to not automatically fallback to prefiltered cubemap for the skydome.

See https://github.com/playcanvas/engine/blob/master/src/scene/scene.js#L388