WebXR AR Light Estimation

[Maksims]

WebXR Light Estimation is still in incubation, but already has experimental implementation in Chrome Canary 84 on Android, under the chrome://flags#webxr-incubations flag.

It provides estimation of lighting by underlying AR systems.
Current API has a spec for three paths:

1. Reflection CubeMap
2. L2 Spherical Harmonics
3. Most prominent directional light

This PR implements basic interface to access most basic light estimation information: most prominent directional light. And Spherical Harmonics for ambient light.

We do not have support for Spherical Harmonics in engine, sadly. But the is internal shader code, that actually generates SH from IBL cubemap, and uses it as ambient light (exactly what we need). But it is not exposed as API.

Reflection cubemap will come as a separate PR.

New APIs:

// pc.XrManager
app.xr.lightEstimation // interface to access light estimation features

// pc.XrLightEstimation
lightEstimation.supported // true if device supports such feature. This information is only available during started XR session, currently it is not possible to find out if it is supported before starting XR session.
lightEstimation.available // true when light estimation data is available
lightEstimation.intensity // intensity of most prominent directional light
lightEstimation.color // color of most prominent directional light
lightEstimation.rotation // rotation of most prominent directional light
lightEstimation.sphericalHarmonics // Float32Array data, with 27 values, 9 of vec3's. Which represents L2 Spherical Harmonics

lightEstimation.start() // request light estimation information and start updating internal data. It will become available sometime later, and `available` property can be used to find out when, as well as `available` event.
lightEstimation.end() // stop light estimation

lightEstimation.on('available', function() { }); // fired when light estimation becomes available
lightEstimation.on('error', function(ex) { }); // fired when light estimation failed to start

Basic Example:

Initialize:

// start light estimation when XR session started
app.xr.on('start', function() {
    if (! app.xr.lightEstimation.supported)
        return; // light estimation not supported

    app.xr.lightEstimation.start();
});

On each update:

var lightEstimation = app.xr.lightEstimation;

if (lightEstimation.available) {
    entity.light.intensity = lightEstimation.intensity;
    entity.light.color = lightEstimation.color;
    entity.setRotation(lightEstimation.rotation);
}

Applying Spherical Harmonics:

if (lightEstimation.available) {
    material.ambientSH = lightEstimation.sphericalHarmonics;
    material.update();
}

Sample project for test

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