getTileDataAvailable() handling for terrain providers

This is a tile loading optimisation for cameras that start near the globe based on advice from Kevin.  It is currently only implemented for the Cesium terrain provider which knows in advance which tiles are present.
This is a breaking change for folks who implemented `TerrainProvider`, right?

We should provide a default implementation to not break them and perhaps warn if we want to eventually require this.  See the [Deprecation Guide](https://github.com/AnalyticalGraphicsInc/cesium/wiki/Deprecation-Guide).