GPU capture optional plugin

[CedricGuillemet]

Metal and bgfx (with RenderDoc Android/Windows/Linux, Xcode on Mac/iOS) provide API to do GPU frame capture programmatically.
This is needed when a user finds a bug in rendering on a device we don't have or it hard to repro without all the assets/scripts.

We can create a BabylonNative plugin that will do the capture. Letting the use zip the file and upload it somewhere so we can debug it (like a crash dump but for GPU).

This plugin can (should?) be separate from the main repo and is optional. Proposed JS API is simple:

BabylonGPUCapture.StartCapture("NameOfCapture");
...
do rendering
...
BabylonGPUCapture.StopCapture();

Repo readme.md will describe ways to get temporary files from the device.

[bghgary]

Here is an example of something we did to debug XR on iOS: https://github.com/BabylonJS/BabylonNative/compare/master...bghgary:arkit-custom-capture?expand=1

Doing this generically would be great!