gltfio: Provide new loader suitable for remeshing

[poweifeng]

Context

The current division of work between AssetLoader and ResourceLoader does not fit well with algorithms that require remeshing (e.g. flat shading in #6358).

Proposal

• Build a new API that will be useful in these use cases.
• Keep AssetLoader and ResourceLoader for existing clients of the API.

[mbalajee]

Will this enable the proper normal generation in Android?
#7851

[poweifeng]

Will this enable the proper normal generation in Android? #7851

Yes, that's one of the goals of having a new loader.

[mbalajee]

Cool so I guess for Android only change that needs to be done is in the resource & asset loader. I actually tried to make some changes and I still don't see the flat normal shading but the model appears all white. can you please take a quick look & suggest if i should make any other changes? This is one of the blocker for us currently