Fix WebXR Layers example. (#24691)
Fix WebXR Layers example. (#24691)

* Fix WebXR Layers example.

    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.

* Update webxr_vr_layers.html

Co-authored-by: Michael Herzog <f6aeef0c0610dd1547ce61332ac92e282a679af1@human-interactive.org>
mrdoob/three.js,makc/three.js.fork,WestLangley/three.js,mrdoob/three.js,06wj/three.js,06wj/three.js,WestLangley/three.js,makc/three.js.fork