Add GltfLoaderSettings

[robtfm]

Objective

when loading gltfs we may want to filter the results. in particular, i need to be able to exclude cameras.

i can do this already by modifying the gltf after load and before spawning, but it seems like a useful general option.

Solution

add GltfLoaderSettings struct with bool members:

• load_cameras : checked before processing camera nodes.
• load_lights : checked before processing light nodes
• load_meshes : checked before loading meshes, materials and morph weights

Existing code will work as before. Now you also have the option to restrict what parts of the gltf are loaded. For example, to load a gltf but exclude the cameras, replace a call to asset_server.load("my.gltf") with:

asset_server.load_with_settings(
    "my.gltf",
    |s: &mut GltfLoaderSettings| {
        s.load_cameras = false;
    }
);

[alice-i-cecile]

The implementation looks good, and this should definitely exist.

I'd like a bit more documentation on how to use this in practice: right now, it's quite hard to piece the usage together.

[alice-i-cecile]

I think it would also make sense to add a load_lights field to GltfLoaderSettings, but that can be done in another PR :)

[github-actions]

It looks like your PR is a breaking change, but you didn't provide a migration guide.

Could you add some context on what users should update when this change get released in a new version of Bevy?
It will be used to help writing the migration guide for the version. Putting it after a ## Migration Guide will help it get automatically picked up by our tooling.

[robtfm]

added lights, and meshes for completeness.

not sure what makes this a breaking change? anyway i added the struct comment as migration guide, let me know if you want something else.

[alice-i-cecile]

Really nice, thank you!

The associated type on a public trait impl changed, which is technically semver breaking: line 144, which then cascades to the load method. I don't really think a migration guide is likely to be helpful, but we probably shouldn't ship this in a patch release to be safe.

[alice-i-cecile]

@robftm missing import in the example :)

[robtfm]

my pc is even worse at running doctests than i am at writing them ...