Support hevc_videotoolbox in ffmpeg

[fix2015]

Is there any chance to make 'hevc_videotoolbox' work with? Something like that args: ['-i', 'video.avi', '-filter_complex', '[0:v]chromakey=#00ff00[ckout];[ckout]despill=green[out]', '-map', '[out]', '-c:v', 'hevc_videotoolbox', '-allow_sw 1', '-alpha_quality', '0.75', '-vtag', 'hvc1', 'video.mp4']