CubeCamera: Add support for THREE.Layers.

[Mugen87]

Fixed #14804.

I've noticed this when playing around with THREE.Layers and THREE.Raycaster. I think it's more correct if the sub-cameras use the same layer object otherwise layer configuration won't work.

[Mugen87]

@munrocket Any ideas why webgl_kinect tends to fail in the regression tests? I'm sure it's unrelated to this PR since the example does not use a cube camera.

[munrocket]

@Mugen87 tried to bring determinism in video tag, but my the only solution was to increase number of attempts. We added css3d_youtube in ignore list for same reason.

I tried also beginFrame API but it fails in Three.js repo #18732.

[Mugen87]

In this case, I would add webgl_kinect to the ignore list, too.

[munrocket]

This picture reminds me Picard from Star Trek :D

[Mugen87]

@mrdoob This PR probably got lost. Are you okay with this change? Or could this be implemented in the renderer somehow (by testing for CubeCamera)? The current change encapsulates this logic in CubeCamera which is probably more straightforward.

[mrdoob]

It sure got lost!

[mrdoob]

Thanks!