Improve glTF material export code

[bghgary]

See https://forum.babylonjs.com/t/texture-wangle-counld-not-save-in-glb/35207/4 for context. This only addresses the first two bullets.

There are a bunch of issues in the existing exporter code:

1. Textures were being de-duped based on the name.
2. Baking texture transform is not actually a valid solution when texture transform cannot fit the KHR_texture_transform extension.
3. Exporting textures go through an unnecessary step of encoding from array buffer to base64 and then back. This is dumb.
4. Code was a mess. Some weird async constructs and code duplication.

Changes:

• Log a warning instead of baking the texture when the texture transform isn't center at the origin.
• Rewrite a chunk of the material export code to de-dupe textures based on whether the texture points to the same internal texture (and mime type).
• Factor the code.

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[sebavan]

LGTM but a lot could go wrong :-) it is funny it does not seem to impact tests ?

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