This release is almost ready.
I figured out how to workaround a key Apple Metal bug and now Metal rendering works on AMD and Nvidia GPUs too! With Mac OS Sierra, the Metal rendering has great performance.
Metal can render on 2 GPUs at the same time. For instance, the 2013 Mac Pro with its twin GPUs renders almost twice as fast.
You can also combine rendering using both Intel Iris Pro and AMD GPUs on a 2015 Macbook Pro. Tested on Mac OS El Capitan 10.11.6.
Both GPUs: 159.75 Mips 8.5X
AMD: 93.14 Mips 4.9X
Intel Iris Pro: 84.64 Mips 4.5X
CUP (4 core): 18.84 Mips 1X
Bug Fix: Fixed image gamma for Resume renders (restart render to add higher quality)