Terrain shader support for water planes smaller than a scene


Currently the terrain shader makes the assumption that a water plane covers the entire map, so things like small lakes within a map will cause terrain throughout the map to assume it is underwater if it is below the elevation of the lake.
We need to make terrain pixel-shader check if it is actually under the bounds of the water plane.
An optimization that we should make it also to only call the terrain shader technique that handles water for chunks of terrain that are within the bounds of a water plane.