Added support for basisu extension to GLTF2 importer

[Promit]

A GLTF2 file contains a textures array, which references items from the images array and the samplers array:
{"sampler":0,"source":0}
Assimp follows these references to lazy load the corresponding image and sampler. Simple enough. But when you're dealing with a GLTF2 that uses Basisu textures, the texture item comes out like this instead:

{
            "sampler": 0,
            "extensions": {
                "KHR_texture_basisu": {
                    "source": 0
                }
            }
        }

So the regular code flow doesn't work and the extension needs to be checked. I've put in a relatively straightforward addition here that recognizes the alternate source and causes Assimp to follow the references correctly.

Fixes issue #4648 .

[Promit]

There is a similar change needed to support the Microsoft extension for DDS textures in GLTF2 (MSFT_texture_dds) but that extension hasn't been recognized in Assimp at all and I thought it better to start with this relatively simple fix. I think it would be worth adding the MSFT extension as well.

[kimkulling]

Looks fine.

[kimkulling]

You need to update your branch.

[alexrp]

@kimkulling I don't know if you're aware of this, but FWIW, GitHub introduced a feature somewhat recently that allows you to easily update a PR that's out of date with respect to the base branch. You just need to go to the repo Settings tab and enable this:

Then PRs will show this:

(That said, if that doesn't work for some reason, I can rebase and resubmit this PR.)