Make replace ABM optional, disabled by default

-> Don't always drop the main mushroom (allowed endless spore generation loop before)
-> Drop less spores, before you got lots of spores

Code re-arranged and clarified

More complex distribution of dry grasses
Remove water below glacier rivers
Gravel as glacier seabed
Do not force-place acacia root
Tune acacia tree density
Match mgv5/v7 mushrooms to tree density
Spawn mgv6 mushrooms by trees

It should alias the old name (mese_block) to the new/current name (mese) rather than the other way round.

Update

Speed optimise: Trunk placing, add_pine_needles, add_snow