Mgv7 spawn search: Cope with extreme custom terrain and biome 'dust'

Mgv7 spawn search: Cope with extreme custom terrain and biome 'dust'

Previously, maximum spawn level was set to 'water_level + 16'. This would result
in spawn search failing if terrain had been customised to be much higher than
'water_level' at all points.

Also raise spawn level by 1 node to avoid spawning half-buried in a biome 'dust'
node such as 'default:snowblock'.