WebXR depth sensing API

[Maksims]

Implements WebXR Depth Sensing API: https://github.com/immersive-web/depth-sensing/blob/main/explainer.md
Which is available in Chrome 87 (Beta Release) behind chrome://flags/#webxr-incubations flag.

It provides two paths (CPU and GPU) to access depth information which is provided by underlying AR system.

New APIs:

// pc.XrManager
app.xr.depthSensing // interface to access depth sensing information

// pc.XrDepthSensing
depthSensing.supported // true if depth sensing is supported
depthSensing.available // true if depth information is currently available
depthSensing.width // width of depth texture or 0 if not available
depthSensing.height // height of depth texture or 0 if not available
depthSensing.texture // texture with packed depth information
depthSensing.uvMatrix // matrix to transform depth texture to normalized space

depthSensing.getDepth(x, y); // (CPU path): get depth information using x,y in depth texture space. Returns value in meters or null if depth information is not available.

depthSensing.on('available', function() { }); // fired when depth information becomes available
depthSensing.on('unavailable', function() { }); // fired when depth information becomes unavailable
depthSensing.on('resize', function(width, height) { }); // fired when depth texture been resized, so uvMatrix needs to be updated on related materials

Test projects:

CPU Path: https://playcanvas.com/project/732030/overview/ar-depth-sensing

It implements CPU path to test for depth in middle of the screen, and allows to plant grass (like hit test example), but works way more reliably than hit test path.

GPU Path: https://playcanvas.com/project/738358/overview/ar-depth-sensing-texture

It implements GPU path by accessing depth sensing texture, and rendering it on top of the camera view, with shader that unpacks depth information and represents it as a gradient (red > yellow > green > cyan) which is distance from 0 to 8 meters.

Depth Occlusion: https://playcanvas.com/project/738703/overview/ar-depth-sensing-occlusion

Demo: https://playcanv.as/p/UN0z1XE2/ (don't forget to enable chrome://flags/#webxr-incubations)
This demo has few things: cpu path object placement, gpu path occlusion using shader and light estimation.
Link to video: https://twitter.com/mrmaxm/status/1329151944540254208

I confirm I have signed the Contributor License Agreement.

[Maksims]

Chrome 87 just has been released to Android. So it works behind the WebXR Incubators flag in release Chrome.

Cl seems to be failing here, with "Setup Java JDK" errors. Not sure how to fix it.

[Maksims]

There have been recent changes to Depth Sensing API, based on community provided feedback.
This PR will have to be updated accordingly once changes are incorporated to the browsers.

[willeastcott]

Hey @Maksims, can you resolve conflicts in xr-manager.js please.