Update Spring 2018
The OpenCL driver for AMD is working again. AMD Metal driver does not correctly render final transforms.
Its now 7 months of broken OpenCL drivers for the AMD GPU in the late 2016 Touch Bar MacBook Pro. (Since Sierra 10.12.4. Affects all builds of High Sierra.)
Metal drivers work correctly for the Mac’s AMD GPU, but some fractals (10 to 20%) render incorrectly. So I don’t recommend Metal as an alternative. (Note: this incorrect render output is a new problem. Older Metal driver versions did not have this problem. The drivers in Q1 2017 worked so well. Now it is Q4 2017.
The legacy of Apple’s unusable GPU compute drivers goes on and on.