Validation errors are very generic

[prideout]

I have a tentative commit that addresses this issue, just thought I'd post an issue before posting the PR.

In Filament we use cgltf_validate in debug builds and it's great for triaging glTF issues that are actually issues in the asset rather than an issue with Filament.

It would be nice if cgltf_validate printed a unique message for each type of failure but I don't think we should bloat the cgltf library with lots of strings that need to be maintained.

The solution I have adds a little macro that lets you optionally trigger an assert when a validation failure occurs. If the macro is defined to assert(false), then you'll see a line number that tells you what specific condition triggered the failure.

[prideout]

For example, my PR will change cgltf_validate from this:

if (mc->mode == cgltf_meshopt_compression_mode_triangles && mc->count % 3 != 0)
{
	return cgltf_result_invalid_gltf;
}

to this:

CGLTF_ASSERT_IF(mc->mode == cgltf_meshopt_compression_mode_triangles && mc->count % 3 != 0, cgltf_result_invalid_gltf);

By default the behavior will be exactly as it is today, but users will now have a compile-time option that adds in an assert.