Support for KHR_texture_basisu in bevy-gltf

[mholiv]

Objective

At this time fully spec complaint gltf files with universal-basisu textures are not loadable in bevy. This PR fixes that by introducing the concept of document image indies.

We add some indies infrastructure and use that to check for basisu as per the spec and prefer it over the native (i.e. strictly png and jpeg) texture.

We also allow for no native gltf texture while we have a universal-basisu texture.

This approach is, can be expanded to allow for the correct parsing of EXT_texture_webp and MSFT_texture_dds (auto rejected it when presented by upstream gltf-rs)

This PR is scoped to strictly fix basisu. A trivial patch could be submitted building on this work to fully correctly parse webp as per the EXT_texture_webp spec.

Background

Right now we can parse gltf files with webp and basisu textures but only if we deviate from the specification. This patch does not act on that.

This patch allows bevy to parse strictly complaint gltfs in addition to how we handle them now.

[mholiv]

This was tested on my local machine and I could load strictly correct gltf files with basisu textures

[ChristopherBiscardi]

looks like the ci failure is just some clippy lints at the moment

[ChristopherBiscardi]

this will also need to be targeted at the main branch instead of release-0.19.0

[mholiv]

This was replaced by #24397 24397