Error loading gltf with webp textures

## Bevy version

```toml
bevy = { version = "0.16.1", features = ["jpeg", "webp"] }
```

## What you did

Exported a scene from Blender with webp as the texture format

## What went wrong

The scene fails to load in Bevy

## Additional information

```
2025-07-21T09:39:13.762892Z ERROR bevy_asset::server: Failed to load asset 'models/tile.gltf' with asset loader 'bevy_gltf::loader::GltfLoader': invalid glTF file: invalid glTF: textures[0].source: Missing data; textures[1].source: Missing data; textures[2].source: Missing data; extensionsRequired[1] = "EXT_texture_webp": Unsupported extension;
```

After reading #11350 I guess this is not really supported, although the presence of [the webp feature](https://lib.rs/crates/bevy/features#feature-webp) gave me the impression it was.

Start with the bevy_gltf::loader::GltfLoader entry point and read issue #11350 for the existing context around glTF texture support. Reproduce the Blender-exported scene with Bevy 0.16.1 and the jpeg/webp features; done requires a clear resolution for EXT_texture_webp loading or a documented explanation of the feature's scope.

Written by the indexing model from the issue text.