webaudio

See:
https://forum.babylonjs.com/t/can-gltf-export-a-specific-mesh-from-scene/10641/3

Repro:
https://www.babylonjs-playground.com/#YBEKDT#4

Expected result:
Fan materials and textures should not be included in exported glTF from this playground.

There should be some basic automated tests for the gamepad support added in #164. I don't have a gamepad, and I imagine most contributors won't, so it'll be very easy to cause regressions. It is also impossible for me or other contributors to refactor/touch that code because we can't test it.

Once there are some tests, there are some refactorings I have noted in #164 that are probably worth doi