Tunnel 3D noises are only calculated when solid terrain is present
in mapchunk, avoiding large amounts of unnecessary calculations
Change 'int' to 's16' in calculateNoise
Change 'i' to 'vi' for voxelmanip indexes for consistency
Keep 'u32 index3d' local to a smaller part of tunnel code
Mgv7: Don't call CaveV7 if no solid terrain in mapchunk
Give 'open' bool a more descriptive name
Flag is set by default in MapgenParams
The global 'trees' flag remains but is now
undocumented and unset by default in MapgenParams
Add mgv6_spflag 'trees' set by default in
defaultsettings.cpp to affect new worlds only
This is automatically backwards
compatible for existing worlds
