Add support for equirect layer

[vincentfretin]

Description:

Add support for equirect layer, mono and stereo 180 and 360 videos and equirectangular image.

Remaining to do:

• Add an example of 180 stereo video, 360 stereo video and mono and stereo equirectangular image, probably use the videos from https://immersive-web.github.io/webxr-samples/layers-samples/eqrt-layer.html (180 video) and from https://immersive-web.github.io/webxr-samples/layers-samples/media-layer-sample.html
• sphere geometry with uv for mono and stereo 180 and 360 video (code from https://github.com/c-frame/aframe-stereo-component) for devices not supporting WebXR layers

Changes proposed:

• Add 'mono-equirect', 'stereo-left-right-equirect', 'stereo-top-bottom-equirect' types
• Add is180 property

[vincentfretin]

I see @danrossi also experimented with this on a threejs example in mrdoob/three.js#31019
I'll probably continue working on this on the weekend.

[vincentfretin]

For stereo-top-bottom image, Meta browser version 38.2.0.13.53.728503412 and OS version 76.1024 still wrongly shows the top to the right eye.
For stereo-top-bottom video with XRMediaBinding.createEquirectLayer, the top is correctly on the left eye.
See issue immersive-web/webxr-samples#132 (comment)

[vincentfretin]

The stereo-top-bottom image on a layer should be fixed on the next Meta browser release.

I did another branch https://github.com/vincentfretin/aframe/tree/layer-equirect-wip with a quick and dirty implementation of using the stereo component and some code that I previously had in my project to switch between mono and stereo the spheres uvs and using a single stereo-top-bottom image, that works but I needs to refactor that to keep only the layer component.

[vincentfretin]

The stereo-top-bottom image on a layer should be fixed on the next Meta browser release.

It's working correctly on Meta browser 38.4!