Write local register cache modifications to Datapixx immediately, then read back
Datapixx register snapshot to local cache. All Datapixx Set* functions do fast
writes to a local register cache on the host, and all Datapixx Get* functions do
fast reads from this local register cache. RegWrRd writes these cached register
modifications back to the Datapixx over USB, then uploads over USB a fresh
snapshot of the Datapixx registers into the local cache.