Restore backwards-compatible logic paths in KHR_texture_transform

[alexchuber]

For reference on what was previously there, see https://github.com/BabylonJS/Babylon.js/blame/83125ed754242d58f5637bde58fb2b5c26ac552a/packages/dev/serializers/src/glTF/2.0/Extensions/KHR_texture_transform.ts.

Note that preExportTextureAsync does NOT prevent the export of "noncompliant" textures, just warns about them. The only thing preventing their export would have been in postExportTexture, canUseExtension.

While it is probably more correct to prevent export when vAng or uAng are present, since we cannot match the visual output, let's prioritize backwards compatibility.

[bjsplat]

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[bjsplat]

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[bjsplat]

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