GEGL should be more careful selecting the opencl platform
On systems with more than one OpenCL platform, GEGL picks the first one, even if that one doesn't provide image support. (This is a realistic scenario: I've an AMD Polaris 11 card with Clover and ROCm platforms installed on Debian/testing. Clover ends up as the first platform but does not provide image support, while ROCm does.)
We need some code like this patch gegl-image-support.patch. It makes GEGL pick the first platform with image support. It works for me. Feedback welcome.