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.
