[result = ] Eyelink(‘WaitForModeReady’, maxwait)
After a mode-change command is given to the EyeLink tracker,
an additional 5 to 30 milliseconds may be needed to complete mode setup.
Call this function after mode change functions to wait until they are finished
maxwait is in milliseconds – max duration to wait for the mode to change.
Returns 0 if mode switching is done, else still waiting (assume a tracker error