game: give adult some acceleration steps from standstill

When navigating tight situations the existing instantaneous 0-100
is difficult to control.  By introducing a small bit of
acceleration, one can pulse the controls to make small movements.

Note this doesn't introduce a deceleration curve, movement still
stops immediately upon release of the controls.  It's not
anything like a physics simulation, it's just a few frames worth
of ramping up to 100% movement speed for the adult, plain linear
ramp for now.

Now it feels like pixel-accurate collision detection might be
worth adding...

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