Fixed bug in fix{Freeze,Melt}

Since glass does not reduce the light level the height map does not
give us the highest non-air block, if that block is glass, because the
height map saves the block with the highest Y value that could get full
sunlight.

This will slow down fixing the weather conditions, but now snow can not
fall through glass.
Merge branch 'master' into minecraft-1.7
New weather condition fixing code.

Fixing the weather conditions is now based on a snow line, i.e. the
highest Y value that can have snow naturally. That way we can simulate
the snow/ice placement of Minecraft 1.7.