Mgv7 floatlands: Various improvements

Mgv7 floatlands: Various improvements

Floatland base terrain underside was too thin, causing excessive water
leakage through tunnels under lakes, now make it thicker.
Floatland mountain terrain had a rim 1 node thick which made it bare
stone, now make it 2 nodes thick to merge with the floatland base
terrain rim and to have a layer of biome material.
Make mountain terrain more exponentially shaped by altering the
exponent.
Remove unnecessary and potentially ugly MYMAX() applied to
n_base_height.