Ensure OpenCL/CUDA is catered for

OpenCL mightn't come up that often but we should still ensure we can cope with it by providing
at minimum `beignet` as a backup, working in upstream snapd to expose the OCL ICD definitions, send a patch to Khronos to support a new `OCL_ICD_FILENAMES` variable and finally ensure snapd also makes the `cuda` libraries available.

Inside this snap we'll also need to add the OCL ICD loader.