mac-videotoolbox: Refactor implementation

Existing code made use of macros which broke parsing in Xcode and
complicated debugging as crucial code was executed within macros and
not actual source code.

Use of mutable CoreFramework data structures (which were never mutated)
is replaced by use of faster non-mutable variants.

When an allocator needs to be used, `kCFAllocatorDefault` is specified
explicitly.

VideoToolbox session properties are also set in bulk instead of
multiple consecutive calls.

Also simplifies detection of Apple Silicon hosts to make code more
readable.