MGLGeometry structs should be boxable

[1ec5]

The structs MGLCoordinateSpan, MGLCoordinateBounds, and MGLOfflinePackProgress should be marked with the objc_boxable attribute so that they can be boxed in Objective-C. For example:

MGLCoordinateBounds cb = …;
NSValue *value = [NSValue valueWithMGLCoordinateBounds:cb];

becomes:

MGLCoordinateBounds cb = …;
NSValue *value = @(cb);

/cc @boundsj

[1ec5]

Fixed in #9343 on the release-ios-v3.6.0-android-v5.1.0 branch.