GLTFLoader: Add support for EXT_texture_webp.

[donmccurdy]

Adds support for WebP images in glTF models.

https://caniuse.com/webp

[takahirox]

I would like to test (and review). Do you or someone else have glTF file including EXT_texture_webp extension?

[donmccurdy]

Sure, here are two examples:

CesiumBoxWebP.zip

• CesiumBoxWebp.only.glb: Uses only its WebP texture, and fails in browsers that don't support WebP.
• CesiumBoxWebp.fallback.gltf: Uses PNG fallback for browsers that don't support WebP.

^Ideally the "fallback" PNG would not be bundled in the same file as the WebP texture, so you only have to download one or the other, but this is a quick test. :)

[takahirox]

Thanks, let me try tomorrow.

BTW, CI test fails but the root issues seem not in this PR but #20540 and #20542.

[arpu]

can https://aomediacodec.github.io/av1-avif/ be included in the same way?

[donmccurdy]

I don't think anyone has proposed a third-party extension for AVIF yet. The browser support is pretty limited, as well: https://caniuse.com/avif.

[takahirox]

I commented two nits, but this change is looking good to me and I confirmed it works on my Windows Chrome and Firefox.

[donmccurdy]

Updated to address feedback, thanks @takahirox and @zeux!

[mrdoob]

Thanks!