Improve DEM-Layer

Improve DEM-Layer

- the old version just draws fixed-size boxes in any zoom. in high zoom
 modes, this results in sparse boxes all over the screen. the new
 version scales these boxes so they fill the area completely. this
 works for both absolute and height gradient

- change of DEM source path from /srtm/version2_1/SRTM3/OLD/ to
 /srtm/version2_1/SRTM3/. again, yes. that's because the first is not
 working but the second is.

Signed-off-by: Guilhem Bonnefille <guilhem.bonnefille@gmail.com>