FlipTimingWithRTBoxPhotoDiodeTest: Make Flip vs. box reporting more intuitive.

Report difference between Screen('Flip') stimulus onset timestamp and
externally measured "ground truth" timestamps (e.g., Datapixx) as

delta = Flip - Measured, instead of delta = Measured - Flip.

This way, zero error means perfect agreement, a positive error
means that Flip reports a too late onset time (false deadline miss),
a negative error means Flip reports a too early onset.

Somehow this needs less brain cycles to attach meaning to than
the case with flipped signs. It is also consistent with the
quick and dirty VBLSyncTest + Datapixx output.