VSM + mipmaps asserts with the vulkan backend

On macOS (not tested elsewhere), using gltf_viewer:

```samples/gltf_viewer -a vulkan```

and activating VSM + mipmaping leads to an assert:

```
-[MTLTextureDescriptorInternal validateWithDevice:]:1357: failed assertion `Texture Descriptor Validation
MTLTextureDescriptor mipmapLevelCount must equal 1 for multisample textures.
```

This works on both GL and Metal. It could be a bug on our side or a problem with MoltenVK.