Fix WebXR Layers example.

[sigmaxipi]

Invisible dummy meshes relied on old alphaWrite logic so they became
opaque after dfca2bd4748b00245225c9e4d8ba0ef7374c9b36 on Jan 27 and
obscured the quad layers. This switches to using colorWrite=false
logic to properly create holes in the 3D scene and let the dummy
meshes interect the controller ray without obscuring the quad
layers.

Tested:
  * https://raw.githack.com/sigmaxipi/three.js/webXrLayersFix/examples/index.html?q=webxr#webxr_vr_layers Properly shows the charts on the top two quads on a Quest 2 with chrome://flags/#webxr-layers enabled and ray intersection works properly with the quad layers and GUI.

Related issue: #XXXX

Description

A clear and concise description of what the problem was and how this pull request solves it.

This contribution is funded by Example

[Mugen87]

https://raw.githack.com/sigmaxipi/three.js/webXrLayersFix/examples/webxr_vr_layers.html
https://threejs.org/examples/webxr_vr_layers

[Mugen87]

I've tested your PR with a Quest 2 but it seems the result is not as expected.

With the prod version, I see the two white rectangles over the snellen images but the rest of the scene is rendered correctly.

With your version, the lil-gui interface becomes black and the white rectangles become black, too, with white borders.

[Mugen87]

With your version, the lil-gui interface becomes black and the white rectangles become black, too, with white borders.

It seems an update of the quest browser solved the issue. The example looks good now.

[mrdoob]

Thanks!