Textures without fallbacks require extensions

Any texture that fails to supply a `source` image should (must?) supply at least one *required* extension.

For example:

```
"textures": [
{
"sampler": 0,
"extensions": {
"KHR_texture_basisu": {
"source": 2
}
}
},
```

The above block implies that `KHR_texture_basisu` (or whatever extension may be used there) should be listed under `extensionsRequired` because otherwise there is no texture `source`.

/cc https://github.com/KhronosGroup/glTF-Sample-Models/pull/285

It might also be good to flag if the source is missing and there aren't any extensions:

```
"textures": [
{
"sampler": 0
},
```

Start with the two texture examples in the issue and trace how the validator handles textures without a source, extensions, or extensionsRequired. Determine the expected diagnostic for each case, then verify the behavior against the examples and add or update validation coverage where the project’s existing tests for these cases are located.

Written by the indexing model from the issue text.