Darktable Snap package and opencl

…and i don’t know anything about snap packages. but to be able to ship opencl enabled binaries, we dynamically dlopen() all library functions manually (i.e. we don’t link against the library, which would mean we’ll have a compile time dependency). this means you should be able to just run dt on a system with libOpenCL.so and the stuff around it and it should go and pick it up.