Implement support for EXT_texture_webp

[bobbens]

Specification:
https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Vendor/EXT_texture_webp

[bobbens]

Oops, it's EXT_texture_webp, not KHR_texture_webp.

[zeux]

This needs to be structured like KHR_texture_basisu; the current implementation for example doesn’t support specifying both png and webp images. Also missing cgltf_write support.

[bobbens]

This needs to be structured like KHR_texture_basisu; the current implementation for example doesn’t support specifying both png and webp images. Also missing cgltf_write support.

How does it look now?

[zeux]

Oh, one more thing - this needs a call to CGLTF_PTRFIXUP in cgltf_fixup_pointers for the newly added pointer field.

[bobbens]

I added a call to CGLTF_PTRFIXUP in cgltf_fixup_pointers. Hopefully should be fine now.

[zeux]

And one more thing 😅 I think this is missing writing the appropriate extension name in cgltf_write_extensions, so a written file with webp extension will be considered invalid by glTF spec (although this would not necessarily interfere with loading it).

[bobbens]

Lots of things I missed when grepping. Completed now perhaps.

[zeux]

Thanks! I've confirmed the parsing works in gltfpack (can't confirm cgltf_write changes since gltfpack doesn't use this but should be good on inspection). cc @jkuhlmann