Add an updateSize method

We need a non-private/protected method to call when the map viewport size changes.

This should also be made part of the API, so third-party code can change it when something else is responsible for a viewport size change that we can't detect.

The (upcoming) full-screen control should also likely call `updateSize` to handle the case where the window size doesn't change when entering full screen mode.  In Chrome, you can start in presentation mode and click the control to go in full-screen mode.  This might be considered a degenerate case.  But the point still stands that 3rd party code will have to tell the map to recalculate its viewport size.