Improvements to texture export functionality to handle compressed textures

[mvaligursky]

Improvements to texture conversion code to canvas, which is internally used by Usdz and Gltf exporters. If the texture is not natively supported by the browser, it gets rendered by a simple shader to render target (which decompresses it from Basis or other compressed formats). We extract pixels from this render target and construct a canvas from it.

Note that this function has internally been made async and returns a promise, to in the future allow async getPixels call.

Additionally, the exporters themselves were update to use this modified function as async, so their internal flow has been changed a bit to handle this.

Fix: originally, those public export functions were incorrectly declaring return type as ArrayBuffer. This has now been fixed to declare it as Promise<ArrayBuffer> as that is what they are returning.