Asynchronous shader compilation in Babylon Native

[glangstonb]

This code supports interfacing with a feature I wrote for Babylon Native allowing asynchronous shader compilation. It's intended to be reverse-compatible with Babylon Native as of commit 08cec4cacf4d187eb839b6318a7414860f8fde77 on master, but that version of Babylon Native doesn't have the asynchronous shader compilation yet; I will make another PR into Babylon Native with the necessary C++ code to support asynchronous shader compilation, and the shader compilation will be asynchronous only when both updates are used together.

Here's the issue on the Babylon Native side for this feature:
BabylonJS/BabylonNative#402

The PR for the Babylon Native side has now been opened:
BabylonJS/BabylonNative#1209

[bjsplat]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[sebavan]

LGTM, thanks !!! but I am no native expert so let s wait on @bghgary review.

[bghgary]

Looks good except for the error handling.

[bjsplat]

Snapshot stored with reference name:
refs/pull/13587/merge

Test environment:
https://babylonsnapshots.z22.web.core.windows.net/refs/pull/13587/merge/index.html

To test a playground add it to the URL, for example:

https://babylonsnapshots.z22.web.core.windows.net/refs/pull/13587/merge/index.html#WGZLGJ#4600

Links to test babylon tools with this snapshot:

https://playground.babylonjs.com/?snapshot=refs/pull/13587/merge
https://sandbox.babylonjs.com/?snapshot=refs/pull/13587/merge
https://gui.babylonjs.com/?snapshot=refs/pull/13587/merge
https://nme.babylonjs.com/?snapshot=refs/pull/13587/merge

To test the snapshot in the playground with a playground ID add it after the snapshot query string:

https://playground.babylonjs.com/?snapshot=refs/pull/13587/merge#BCU1XR#0

[bghgary]

LGTM