Fix Velocity() calculation for BulkSplineEvaluator.
Fix Velocity() calculation for BulkSplineEvaluator.

We needed to factor in playback rate. If playing back at 2x,
the slopes will be 2x as steep.

Tested: Sample game plays fine on Mac.
Change-Id: I4dfefc3dc92c4b5eb76de43b61f36f5948391b6a

google/motive,google/motive