Metal tvOS 15.0 Beta 2 to Beta 3 API Differences

MTLDevice.h
Removed -[MTLDevice newLibraryWithDescriptor:error:]
Removed -[MTLDevice newLibraryWithDescriptor:completionHandler:]
Added -[MTLDevice newLibraryWithStitchedDescriptor:error:]
Added -[MTLDevice newLibraryWithStitchedDescriptor:completionHandler:]