## 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.
