Blender Git Loki

OpenCL: Bring back CYCLES_OPENCL_TEST override

Back in 2.79 you could either use the debug panel or an
environment variable to override using OpenCL for unsupported
hardware. Which was rather useful for developers when testing
on NVidia just to be sure the CL kernels at-least build properly.

This broke in rB949ab753bb2

This diff restores testing though the CYCLES_OPENCL_TEST
environment variable.

Differential Revision: https://developer.blender.org/D7202

Reviewers: brecht