Create a CAMetalLayer-backed NSView/UIView and attach it to the specified window.
SDL_MetalView SDL_Metal_CreateView(SDL_Window * window);
On macOS, this does not associate a MTLDevice with the CAMetalLayer on its own. It is up to user code to do that.
The returned handle can be casted directly to a NSView or UIView. To access the backing CAMetalLayer, call SDL_Metal_GetLayer().