Some thoughts i had in the midlle of the night

Now opencl_find_gpu is called only if there is (at least) one valid OpenCL device (previous patchs). So opencl_find_gpu must point to one valid OpenCL resource (even if it is a CPU). It means opencl_find_gpu will point:
- to an OpenCL CPU.
- or better, to an OpenCL GPU (if any).

It must be renamed and static, I think.