Generate different LOD of height map at runtime into one texture

The LOD level is controlled by view position.

The generated height map is read from same sized(latitude and
longitude) aquare unit of tiff image which contains DEM data.
In the current implementation, the image is downloaded from
EarthExplorer: http://earthexplorer.usgs.gov/
With SRTM data set in TIFF format.

Signed-off-by: plysan <plysab@gmail.com>