glTF loader for api samples is severely limited

[SaschaWillems]

While working on a new sample requiring me to render a complete glTF scene I noticed that the glTF loader for the api samples is severely limited. It can only load (and display) a single mesh with a single primitive from a glTF file. That's a real hindrance and something we should address near-term. It also does not apply any matrices, so scenes aren't scaled properly.

Sadly the glTF loader initially taken from my code has been partially wired to the framework, making it non-trivial to fix this without needing to change a lot of things.

[SaschaWillems]

As a workaround one can go iterate over the meshes in vkb::sg::Scene. But that then requires lots of manual things do be done explicitly in a sample (materials, matrices, etc). and is something we could simplify to open this option to all api samples.

[SaschaWillems]

But even using the workaround mentioned above: the current glTF loader in the framework crashes with some very basic glTF files (only few static meshes, no images, etc.) that work fine in my samples. So this is something we should tackel.