core: initial implementation of user-settable xintensity. Has a bug.

Changing `intensity` and to it from `xintensity` works fine.

Changing `xintensity` sometimes fails to enqueue kernel.

For example, starting with --xintensity=128 (on a 5850) and
then changing to 64, 42, or 100 is reliable. However, changing to 127 is
not, and produces

    [04:13:01] Error -54: Enqueueing kernel onto command queue. (clEnqueueNDRangeKernel)
    [04:13:01] GPU 0 failure, disabling!

Manually enabling the disabled GPU is successful, but the GPU no longer
submits shares.

This might be a hardware limitation.