Additional restrictions on alpha modes usage

From https://github.com/BabylonJS/Babylon.js/issues/3155
> shouldn't the 1.0 alpha in baseColor (absent either diffuse texture or vertex colour) essentially make BLEND meaningless?

Let's specify all valid combinations of `material.alphaMode` and alpha channel in
* `material.pbrMetallicRoughness.baseColorFactor`
* `material.pbrMetallicRoughness.baseColorTexture`
* `KHR_materials_pbrSpecularGlossiness.diffuseFactor`
* `KHR_materials_pbrSpecularGlossiness.diffuseTexture`
---

#### Proposal
- `OPAQUE`: Texture shouldn't contain alpha channel and factor's alpha value should be `1.0`.
- `MASK` & `BLEND`: Texture should contain alpha channel or factor's alpha value shouldn't be `1.0`.

#### Questions
- When reporting on invalid assets, what should issue's pointer be? Whole `material`, `material.alphaMode`, or specific textures/factors?
- What if MR material is OK, but SG isn't (or vice-versa)?
- What about vertex color's alpha channel?

/cc @bghgary @zellski @sebavan

Start by reviewing the proposal and the linked glTF issue, focusing on alphaMode with the listed metallic-roughness and specular-glossiness factors and textures. Resolve the open questions about invalid-asset pointers, mixed material validity, and vertex-color alpha before identifying implementation entry points. Done means the validator consistently enforces the agreed combinations and reports invalid assets at the chosen locations.

Written by the indexing model from the issue text.