Prevent long terrain credits from being cut off

As discussed offline, terrain providers can currently only return a single credit, therefore long credits get get off if the window is to small or the credit to long.

The short term solution is to remove the `white-space: nowrap;` from `cesium-credit-text` CSS class.  The longer solution is to allow terrain providers to return multiple pre-broken up credits that can then be laid out using nowrap.