Made the air resistance constant sane.
Made the air resistance constant sane.

The previous constant used the velocity_scale_factor too many times
   (now it only uses it once, since gravity_acceleration_magnitude
   contains it once)
   and, related to that, was just way too high; with that value, air
   resistance didn't even have the slightest affect until a tile was
   *already* moving at about half-a-tile-per-frame downwards.

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