private boolean isInside(int x, int z) {
        if (x < 0 || z < 0 || x > totalSize || z > totalSize)
            return false;
        return true;
    }
is the 2d point inside the terrain?
@param x local coordinate
@param z local coordinate
jme3-terrain/src/main/java/com/jme3/terrain/geomipmap/TerrainQuad.java