fix(USDZExporter): alpha issues in diffuse map

[Patrick-van-Halm-360Fabriek]

Why

When applying a texture with a transparent background, the USDZ exporter doesn't export the transparency of the texture. In this PR it will fix it.

What

I changed the shader of the USDZ exporter to include the opacity as alpha and opacityThreshold. The applied changes are based of the Pull request in THREE,js USDZExporter: Support opacity materials. It looks like this change later was removed from the source code again. I reimplemented it in this code base with an additional step to always include an opacityThreshold of 0.01 when the material is transparent since it will show a white blur if not.

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:tada: This PR is included in version 2.30.2 :tada:

The release is available on:

• npm package (@latest dist-tag)
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