[MGLMapView zoomToSouthWestCoordinate:northEastCoordinate:] not coordinates deterministic

[friedbunny]

Using [MGMapView zoomToSouthWestCoordinate:northEastCoordinate:animated:] produces a different result based on the zoom/scale of the map.

I expected the method to always fit the given coordinates on the screen — instead, it zooms in on their center point at a zoom level calculated based on _mbglMap->getScale(). This often results in z18 and never actually fits the viewport to the exact coordinate bounds.

Calling the method repeatedly with the same coordinates but at different zooms yields this:

Mapbox GL: input-zoom: 13.45   scale: 11161.71     output-zoom: 18.00
Mapbox GL: input-zoom: 15.78   scale: 56414.93     output-zoom: 18.00
Mapbox GL: input-zoom: 3.69    scale: 12.91        output-zoom: 9.18
Mapbox GL: input-zoom: 9.18    scale: 578.48       output-zoom: 14.66
Mapbox GL: input-zoom: 14.66   scale: 25918.91     output-zoom: 18.00

[MGMapView zoomToSouthWestCoordinate:northEastCoordinate:animated:] is only used right now for centering the map on the user location annotation and is somewhat broken for that purpose, too, besides not being a public method. (#1092 tracks what to do about this API decision.)

I would like to use this method for bounds-based syncing in the demo app I'm working on, so I've hacked together an interim solution, but the maths are absolutely wrong:

- (void)fitBoundsToSouthWestCoordinate:(CLLocationCoordinate2D)southWestCoordinate northEastCoordinate:(CLLocationCoordinate2D)northEastCoordinate animated:(BOOL)animated
{
    CLLocationCoordinate2D center = CLLocationCoordinate2DMake((northEastCoordinate.latitude + southWestCoordinate.latitude) / 2, (northEastCoordinate.longitude + southWestCoordinate.longitude) / 2);

    CGFloat scaleX = _mbglMap->getWidth() / (northEastCoordinate.longitude - southWestCoordinate.longitude);
    CGFloat scaleY = _mbglMap->getHeight() / (northEastCoordinate.latitude - southWestCoordinate.latitude);
    CGFloat minZoom = _mbglMap->getMinZoom();
    CGFloat maxZoom = _mbglMap->getMaxZoom();
    CGFloat hack = ([[UIApplication sharedApplication] statusBarOrientation] == UIInterfaceOrientationPortrait) ? 0.5f : 1.f;
    CGFloat zoomLevel = log(MIN(scaleX, scaleY)) / log(2) - hack;
    zoomLevel = MAX(MIN(zoomLevel, maxZoom), minZoom);

    [self setCenterCoordinate:center zoomLevel:zoomLevel animated:animated];
}

I put together a demonstration of this bug and my hack in a branch here. Use the action sheet's "Zoom to Oregon" option to see new default behavior.

/cc @incanus @1ec5

[friedbunny]

My hack inexplicably appears to work very well, I can make it match Google's fitBounds perfectly (although without UIEdgeInsets support). This will be sufficient for Treble, but clearly our bounds fitting code needs cleaning up and somebody to explain/correct the math.