glTF sandbox: alpha=0 with MASK mode not transparent

[scurest]

Repro
In https://sandbox.babylonjs.com/, the following glTF material is opaque

Cube-Alpha0.zip

    {
      "alphaMode": "MASK",
      "doubleSided": true,
      "name": "Material.001",
      "pbrMetallicRoughness": {
        "baseColorFactor": [
          0.800000011920929,
          0.800000011920929,
          0.800000011920929,
          0
        ],
        "metallicFactor": 0,
        "roughnessFactor": 0.5
      }
    }

Because this uses MASK mode and the alpha value (0) is less than the alphaCutoff (default of 0.5), this should be fully transparent.

If alpha=0 is achieved with a fully transparent texture instead

Cube-TextureAlpha0.zip

    {
      "alphaMode": "MASK",
      "doubleSided": true,
      "name": "Material.001",
      "pbrMetallicRoughness": {
        "baseColorTexture": {
          "index": 0  // <- this has alpha=0 everywhere
        },
        "metallicFactor": 0,
        "roughnessFactor": 0.5
      }
    }

then it renders transparent as expected.

Screenshots

Desktop (please complete the following information):

• OS: Arch Linux
• Browser: Firefox
• Version: 128

Additional context
KhronosGroup/glTF-Blender-IO#2328

[Popov72]

cc @bghgary

[bghgary]

We seem to be ending up in alpha blend mode for this case. I will investigate, but we probably need to overhaul how we detect alpha a bit.

[bghgary]

I'm going to try to fix this along with #15504 since it's somewhat related, but this may take a while to confirm everything is working.