Introduced a setter/getter for constrain mode. On iOS and OS X, the zoom level inspectable causes the zoom level to be set independently from the longitude and latitude. Thus, the latitude inspectable had no effect because the latitude was constrained to 0 at z0. Temporarily removing the heightwise constraint allows the map to center on the intended location before zooming, which is the usual case for storyboards and XIBs. On iOS, the only guarantee we have timing-wise is that all the inspectables are applied after initialization but before the view is added to a window. So we reimpose the heightwise constraint as soon as the view is added to a window, that is, before the user has a chance to pan the map out of bounds.
Fixes#3868.
ConstrainMode gives flexibility to our engine to choose between
constraining both vertically and horizontally, or just vertically
(default behavior).
Constrain in both axis means we can no longer pan the map beyond the map
boundaries. This fixes an issue where e.g. annotations disappear upon
crossing the map boundaries.
Adding new mbgl::GLContextMode enum to mbgl::Map ctor, which gets stored
in MapData. In shared GL context environments, we cannot assume that the
GL state that has been left since the last draw is the same, so we reset
the GL configurations to their default values.
Both Android and iOS now use pause/resume. The map thread is
always running, and be able to receive messages. When paused,
it will just refuse to render.
