fixes#838
instead of rendering ad libitum on the map thread, we are now driving rendering from the UI thread on iOS via the map.renderSync() function. There are still white bars during the rotation, but the general content of the view is kept visible.
- upgrades GLFW to 3.1
- removes swapped/needsSwap in favor of a more explicit scheme
- View#invalidate() now replaces View#swap() and is called whenever the View needs to trigger a rerender. GLFW and Android to this right away, while iOS goes back to the main thread and does the Map redrawing as part of the GLKView update
- sets all iOS deployment targets to 7.0
- disables SQLite3 version check, since the library version changed on iOS 8.2
we are now scoping all file requests to an environment object. The FileSource implementation treats
this as an opaque pointer, but allows canceling all Requests that are associated with that pointer.
This is necessary to abort all file requests that originated from a particular Map object. Aborting
a file request is different from canceling a file request: A canceled request doesn't have its
callback called, while an aborted request will have its callback called with an error, indicating
that the environment is going to be shut down.
