fixes #1181: fix lat/lng & zoom ordering problems

[incanus]

There were two things going on here:

• On iOS, we limit the minimum zoom to 3 like MapKit so that we don't ever see world edges or a wrapping, repeating map. However, in various center set routines, we weren't checking that the level was at least 3 before trying to move the map, which could have limited it artificially and failed at setting the center desired. This abstracts out an MGLMinimumZoom constant since we use it in various places anyway, plus ensures that we are at the minimum zoom on iOS before changing viewport properties.
• The second problem was more subtle. In C++, all of the setters and getters for center and zoom queried Map's transform except for getLatLng(), which queried state instead. This could cause things to be out of sync when setting properties one after the other in the same run loop pass.

/cc @kkaefer just to be sure.

[1ec5]

Do these methods need to account for Map::getMinZoom() and Map::getMaxZoom() as well?

[incanus]

Good point @1ec5 — will adjust.

[incanus]

Adjusted for Map::getMinZoom() at each query, even though this is unlikely to change. I don't think we need to worry about max because we're not (yet?) talking about letting the (Cocoa) user cap this and it is meant to be in the 18-to-maybe-someday-20s range.