Add support for WebP extension

[OmarShehata]

This PR adds support for the EXT_image_webp extension. It makes sure to take into account for the image defined in the extension if found when combining images into buffers.

[lilleyse]

Make sure to add a test. Otherwise looks good.

[OmarShehata]

@lilleyse just did! I didn't want to @ mention you until I did 😄

[lilleyse]

Actually, would it make sense to wait on this until a glTF extension is in place? Only png and jpeg are officially supported.

[OmarShehata]

I think that makes sense. I could start working on putting together a draft.

[pjcozzi]

Absolutely please write the extension concurrent to this; if the extension is held up then merge this to a non-master branch. See KhronosGroup/glTF#1503.

[lilleyse]

@OmarShehata Offline we talked about what changes gltf-pipeline would need once the extension is in place. Anywhere images are processed it will need to check for the WebP extension and process the WebP images too. A good way to do this is look at wherever ForEach.image is called.

[OmarShehata]

I've expanded the scope of this PR. It should now correctly process glTF's with the proposed EXT_image_webp extension. I used this branch to generate the test model for the implementation in CesiumJS CesiumGS/cesium#7486

[OmarShehata]

Superseded by #450