Check file headers instead of filename regex to determine image format

This came up in https://github.com/CesiumGS/cesium/pull/9513 - files like `Material.js` have code along the lines of:

```js
var ktx2Regex = /\.ktx2$/i;

function createTexture2DUpdateFunction(uniformId) {
...

if (ktx2Regex.test(resource.url)) {
promise = loadKTX2(resource.url);
} else {
promise = resource.fetchImage();
}

...
}
```

We should update these checks to check .ktx2 header info since not all files will have the .ktx2 extension.

Start with the filename-based KTX2 check shown in Material.js, then trace how loadKTX2 and resource.fetchImage are selected. Done means the relevant image-format checks use KTX2 header information so files without a .ktx2 extension still use the correct loader.

Written by the indexing model from the issue text.