Add test for invalid skinned meshes

[greeble-dev]

Objective

Add a test that would have caught #16929 and #18712.

Solution

The PR adds a test_invalid_skinned_mesh example that creates various valid and invalid skinned meshes. This is designed to catch panics via CI, and can be inspected visually. It also tests skinned meshes + motion blur.

The screenshot shows all the tests, but two are currently disabled as they cause panics. #18074 will re-enable them.

Concerns

• The test is not currently suitable for screenshot comparison.
• I didn't add the test to CI. I'm a bit unsure if this should be part of the PR or a follow up discussion.
• Visual inspection requires understanding why some meshes are deliberately broken and what that looks like.
• I wasn't sure about naming conventions. I put test in the name so it's not confused with a real example.

Testing

cargo run --example test_invalid_skinned_mesh

Tested on Win10/Nvidia, across Vulkan, WebGL/Chrome, WebGPU/Chrome.