Replaced the complex formula for overpanning with a simple one that makes the map feel much slipperier than before. `MGLMapView` now overpans just a little more aggressively than `MKMapView` but still a lot less aggressively than Google Maps.

Fixes #1266.

When the panning velocity is low enough, ignore the pan gesture to simulate friction on the device’s glass screen.

Apply a subtle animated zoom after a pinch gesture completes. Lots of magic numbers involved. Avoid drifting when the pinch is near-static, and when the scale factor would go below the minimum. There is more resistance to pinching outward than pinching inward, because it’s far easier to lose your place when you accidentally zoom out. Disabled two-finger pan, which is typically reserved for changing pitch; otherwise, it’s easy to accidentally pan while intending to pinch.

fmodf() would conflict with the fix for #1199.

It’s pretty darn hard to get up to 5 rad/s with the same two fingers.

Follow-up to cf33edf.