Revisit device filtering

[vmx]

The OpenCL code to get a device could need a refactor, it can likely be simplified.

[vmx]

Related to this issue. We are currently filtering out all devices which are not AMD or Nvidia. This makes the library less general purpose. The reason to do this is, that you want to have a powerful GPU selected by default if one is available and not an integrated card.

I propose that we change the code, so that AMD and Nvidia cards are preferred, but if none are found, any other device that supports OpenCL will be used. This way rust-gpu-tools can also be used in libraries that only need it for testing and don't really need a powerful GPU.

[vmx]

Another idea would be to expose filter capability. This means that rust-gpu-tools would be able to return all available graphic cards (perhaps with a custom sorter order, I propose Nvidia, AMD, Intel, others). One could then request only a subset of those, filtered by vendor. This way one of the upstream dependents, rust-fil-proofs, could request what is best for that library.