Make glTF feature

[kvark]

I noticed that we are spending multiple seconds compiling even some trivial changes. We need to investigate and see if something can be done about this. E.g. we could hide gltf stuff behind a (default) feature, or some derivatives, or use less generics, etc.

[alteous]

I feel I must apologise for the atrocious compile time of gltf. It's become a bit of a joke. 😛

[kvark]

@alteous would you be good with having gltf feature? I think it doesn't even have to be default.

[alteous]

Yes, that's perfectly reasonable. Be aware though the documentation uses gltf in the examples.

[kvark]

@alteous not sure, but perhaps the animation.rs documentation should provide non-gltf examples instead

[alteous]

Although the compile time of gltf is really awful, it would be a shame to not have it as a default feature since it would limit the available 3D formats to just OBJ. Also, although glTF standardises PBR, we don't need to have the PBR shader with the glTF loader since in many cases we can fall back to the Basic material kind.

[kvark]

I don't think having it as a feature would exclude anyone. Adding a simple features = ["gltf"] to Cargo seems simple enough. I see it a candidate for a feature because it's fairly self-contained and has it's own specific external dependencies and examples.

[alteous]

I agree it should be a feature; I'm saying it should be enabled by default.

[kvark]

Let me clarify my position, please:

1. By no means I want to make the current glTF logic unavailable to the users - it's amazing, and I consider it a pillar of three-rs. Not that many projects feature high quality glTF + PBR + animation support.
2. We both seem to agree that gltf needs to be gated a feature
3. If the question raises "should this feature be default", then given all our equals (!), the answer should be "no". Users who don't care will get faster compile times (and run-times, given no need to link some shaders?). Users who do care - can easily switch it on. Unless there is some negative effect on the core function of the project, a feature shouldn't be default.
4. Same can be said about obj, but it's way less code there.

[alteous]

Reluctantly, I'm happy to let gltf be opt-in for the sake of compile times. I hesitate due to conflict of interest of wanting to see glTF as the 'standard' ("jpeg of 3D") runtime format for 3D opposed to other legacy and, frankly, inferior formats.

[kvark]

glTF is certainly not the reason of slow compile times on my machine. Non-gltf examples take 25 sec, while gltf-enabled ones take 30 sec. I'm going to rename this issue and leave #183 for follow-ups.