Registration fails when using AMD OpenCL device on MacOS 10.15

[rg2]

Run the wiki commands for registration and registration replay (https://github.com/rg2/xreg/wiki/Walkthrough%3A-Single-View-Pelvis-Registration), but pass a device ID corresponding to an AMD OpenCL device. Although the commands complete, the results are completely incorrect.

For example:

xreg-regi2d3d-replay regi_debug_example1_1_pd_003_proj0_w_seg.h5 --video-fps 10 --proj-ds 0.5 --ocl-id AMDRadeonPro5500MComputeEngine

First inspection indicates that reads to the 3D texture object (read_imagef()) are always returning zero during ray casting.