Use 2 paths OpenCL vendor files.

%{libdir}/GL/OpenCL/vendors contains files from GL extensions.

%{libdir}/OpenCL/vendors contains files from SDK.

We use %{libdir} for both since we want to make sure multiarch picks up the right file if they were named differently.

When testing on Nvidia, if you have flatpak <1.1.1, then pass --devices=all to flatpak run, otherwise opencl cannot be used.

Fixes #591 (closed).