There is a new API for checking Hw encoder capabilities on macOS 10.13 Beta. We should make use of this to query capabilities instead of trying to create sessions in Mac H264 encoders. 

Doc: https://developer.apple.com/documentation/videotoolbox/2887271-vtcopysupportedpropertydictionar?language=objc
Presentation: https://devstreaming-cdn.apple.com/videos/wwdc/2017/511tj33587vdhds/511/511_working_with_heif_and_hevc.pdf