Support for WebP textures (EXT_texture_webp)

[atteneder]

There's a ratified vendor extension that enables using WebP files as textures.

This can help to reduce transmission sizes.

I've been hesitant to tackle this, since KTX with Basis Universal super compression can help in those situations and has the benefit of transcoding to GPU formats with reduced memory requirement.

But I assume WebP can reach even higher compression ratios/quality.

It's also been requested multiple times now and there's been a rejected PR (see #632).

There are existing solutions to load WebP (Unity.WebP), but nothing audited or provided by Unity itself. Without that I'm against integrating third party packages in a soft dependency style (similar to how KTX and Draco are integrated).

Instead we should extend the image load context and add-on API so that once can inject a custom texture importer. We could then inverse the dependency and provide an (unsupported) WebP injection.

[atteneder]

This has to implemented via #733 .