Mgv7: Add 'mount_zero_level' parameter

Allows setting of the mountain 'zero level' (y where density gradient is zero).

It is easy to vertically shift smooth terrain by editing noise parameter 'offset',
but vertically shifting mountain terrain was complex and imprecise, involving
making a calculation based on an average of the mountain height parameter.

minetest.conf.example

@@ -1397,6 +1397,10 @@
 #    type: flags possible values: mountains, ridges, floatlands, caverns, biomerepeat, nomountains, noridges, nofloatlands, nocaverns, nobiomerepeat
 # mgv7_spflags = mountains,ridges,nofloatlands,caverns,biomerepeat
 
+#    Y of mountain density gradient zero level. Used to shift mountains vertically.
+#    type: int
+# mgv7_mount_zero_level = 0
+
 #    Controls width of tunnels, a smaller value creates wider tunnels.
 #    type: float
 # mgv7_cave_width = 0.09