Fix to pass "Alpha Blend Mode Test"

[anormann1974]

Pull Request Template

Description

Make a fix to successfully pass the glTF-AlphaBlendModeTest. When importing an opaque material, it is needed to set "pbr.alphaMaskCutoff" to zero. Otherwise the default value of 0.5 would be used, which is wrong for an opaque material.

Type of change

Please delete options that are not relevant.

• [x] Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Tested against this model: AlphaBlendModeTest

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [ ] Any dependent changes have been merged and published in downstream modules