PcloudStimulusBuffer::produceFrameReq():
Now correctly produces into the stim frames for the
PcloudIntensityStimulusBuffer object that's attached to the
PcloudStimulusProducer. If there's no attached I stimbuff, then
the OpenCL kernel will simply not write out the intensity data.
This is the first moment when we actually use the SP-MC ringbuffer
properly and actually cycle through the frames, producing into
them one by one.
When mapping in the collationBuff we only need to supply CL_MAP_WRITE
and not CL_MAP_WRITE_INVALIDATE_REGION since we don't care to
preserve the contents of the collation buff as input to the
collation kernel.