MGLMapCamera interprets viewing distance incorrectly

[1ec5]

+[MGLMapCamera cameraLookingAtCenterCoordinate:fromDistance:pitch:heading:] is based on MapKit’s +[MKMapCamera cameraLookingAtCenterCoordinate:fromDistance:pitch:heading:]. But even though the distance parameter is documented as “the straight-line distance from the viewpoint to the centerCoordinate”, it actually controls the altitude parameter, which is the height the viewpoint above the ground plane. This significant discrepancy has apparently always been present in the MGLMapCamera implementation, since #2193. Fixing it will likely require a semver-major release, but in the meantime it might be desirable to add a correct initializer alongside the buggy one.

/ref #12938
/cc @mapbox/maps-ios @d-prukop