Improve LatLngBounds

[jfirebaugh]

Replace:

• LatLngBounds() (constructor)
• LatLngBounds::getExtendable() (static)
• LatLngBounds(sw, ne) (constructor)

With:

• LatLngBounds::world() (static)
• LatLngBounds::empty() (static)
• LatLngBounds::fromCorners(a, b) (static, swaps coordinates as necessary)

[jfirebaugh]

@1ec5 With respect to the above, how should MGLLatLngBoundsFromCoordinateBounds behave? Is MGLCoordinateBounds expected to enforce sw < ne, or be lenient about order?

[jfirebaugh]

@1ec5 Also, if LatLngBounds does not have a default constructor, how do you specific a default initializer for this:

@implementation MGLMultiPoint
{
    CLLocationCoordinate2D *_coords;
    size_t _count;
    mbgl::LatLngBounds _bounds;
}

[1ec5]

MGLCoordinateBounds itself is lenient about order.

As ivars in Objective-C are initialized implicitly, I don’t think it’ll be possible to keep a LatLngBounds as an ivar if the default initializer is removed. In this case, it’d be perfectly fine to have this class hold an MGLCoordinateBounds instead.

[1ec5]

The naming of this method isn’t incredibly obvious. Could you add a documentation comment so its purpose is clear?

[1ec5]

The new cardinal direction getters for LatLngBounds are lovely. 👍