#2329 Constrain map to user specified bounds

[maciekish]

This PR enables the user to constrain movement of the map to a specific area.

Example
Set the bounds to lock the map to Sweden:

[self.mapView setUserConstraintsSouthWest:CLLocationCoordinate2DMake(55.336944, 10.9575) andNorthEast:CLLocationCoordinate2DMake(69.06, 24.155833)];

Implemented in transform_state.cpp by looking at how the existing TransformState::constrain(double& scale_, double& y_) function works to prevent panning the map outside 90 degrees latitude.

The new function TransformState::userConstrain(double& scale_, double& x_, double& y_) will constrain scale, x and y. By implementing it at this level both panning, flicking, pinching, double tapping and tapping the map with two fingers will be correctly constrained to the given coordinates. It does however not take angle or pitch into account.

Requested by me in #2329, but since i don't like waiting i went ahead and did it myself. This is my first c++ code ever so please be gentle :) Any feedback is appreciated.

[maciekish]

Here it is in action http://s.swic.name/d7z3

[maciekish]

Updated with the latest changes from master

[1ec5]

Thanks for this PR. We’re thinking about a different approach to constraining the viewport that relies on a delegate method, similar to the way UIScrollView works: #2457. The nice thing about that approach is that it’ll handle a “rubber band” effect without much effort on the part of the developer. It’ll also be possible to constrain to a non-rectangular region, which may be important if rotation or tilting is allowed.

[1ec5]

Closing for now; will revisit later if #2457 turns out to be inadequate.