Psychtoolbox>Datapixx.{mex*} subfunction

Write local register cache modifications to Datapixx at leading edge of next
video vertical sync pulse, then read back Datapixx register snapshot to local
cache. This is a good way to write Datapixx registers with microsecond
synchronization to the start of a video frame, then pause program execution
until after onset of the vertical sync pulse. 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. RegWrRdVideoSync writes
these cached register modifications back to the Datapixx over USB, then waits
for a fresh snapshot of the Datapixx registers to be returned over USB. The
cache writeback USB message is sent immediately, but the Datapixx itself waits
until the next vertical sync pulse before writing the registers.
RegWrRdVideoSync returns when registers are read back after the vertical sync

###See also: RegWrRd, RegWrVideoSync, RegWrRdPixelSync