Ensure a GLTF Default material is loaded

[4-rodrigo-salazar]

GLTF does not require that mesh primitives provide a material. In that case, a default material is supposed to be used.

In the bevy gltf loader, we iterate over all materials (gltf.materials()) but this iterates over all the materials defined in the gltf, not including the default material. This means we never create a StandardMaterial asset for the default gltf material. This causes a panic later on since we do create a PbrBundle referencing the handle for the default material.

Panic:
thread 'Compute Task Pool (3)' panicked at 'called Option::unwrap()on aNonevalue', C:\github\bevy\crates\bevy_render\src\shader\shader_defs.rs:101:52

This changes makes it so that when iterating over all mesh primitives, if we run into a primitive which has a material which isn't yet in the 'load_context', then we at that time create the standard material and add it to the load_context. The default material will be captured here since primitive.material() will return it. Unfortunately, the gltf library can not provide a default implementation function, otherwise we could just append this onto our gltf.materials() iterator.

Here is a glb file which reproducing the issue (provided by Sheepyhead on discord). I have another glb that repros this as well from my own project, but that one is not postable.

man_with_melting_head.zip

Tested loading this glb and also tested FlightHelmet.

[ColdIce1605]

oh I just realized that this is the issue I'm having as described in this pr

[cart]

Good call. The GLTF default material thing is a little weird, but looking at the gltf lib implementation I think this approach is probably the right call.