see #2400, also removed a now unused alias

Icesheet ice dungeons now have ice stairs.
Remove some now-unnecessary sandstone mapgen aliases, one
remains as it is required by the dungeon loot mod.
Re-arrange mapgen aliases to separate those needed for mgv6.

No longer use the hardcoded engine cave liquids.
Water only in '_ocean' biomes, water and lava in '_under' biomes.

In preparation for biome-defined: dungeon materials, cave liquids,
stone type, ores, decorations.
'_ocean' biomes now extend to y = -255 to be deeper than default
mgv5 oceans, and to create 'shallow underground biomes'.
Remove unnecessary biome lists for gravel and silver sand blob ores,
as those are already defined to only appear in default:stone.

Alter 'large cactus' schematic to place another force-placed cactus node,
to replace the cactus seedling on growth.
Make schematic 5x7x5 to solve rotation, placement and protection check
issues.
Add a y-slice probability for height variation.

Growth time is tuned to not make this a faster way to obtain cactus nodes
compared to normal cactus farming.
Seedling texture by Extex101.

Use sapling/seedling description in protection intersection message in
'sapling_on_place' function.

Precisely reduce trees and logs per mapchunk division to compensate.

Replaces 'bush' in snowy grassland.

These use 'spawn_by' to avoid steep slopes.

Alter a few biome y-limits for consistency.

Move previous tundra to highland and remove the snowblocks that are
unsuitable for a fairly dry biome, use snow slabs instead.

Add silver sandstone strata to 'cold desert' biome.
Add sandstone and desert_sandstone strata to 'desert' biome.

Reduce bronze tool capabilities to be between stone and steel.
Swap depths of iron ore and tin / copper ores for corresponding
progression.

Iron at y = 0 was far too easy to find.
Adjust gold lower region to be twice the depth of highest level, like
all other ores.
Diamond at y = -500 was too easy to progress to.
Make diamond and mese block deeper to create a depth progression from
mese crystal to diamond, to reflect tool progression.
This all creates a satisfying 2^n depth progression, with lower regions
being at twice the depth of highest levels.

Use noises to create a varying mix in coniferous forest biomes:
Areas of large pines only, areas of small pines only, mixed areas.
While also having areas of high and low tree densities.
Saplings grow into large or small pines with equal chance.

Has 'is_ground_content = true'.
Drops 'default:ice'.

Switch y_max and y_min for flowers, fireflies and default

Height 20 to 32 nodes.
Dependent on chunksize >= 5.
Base limited to maximum altitude y = 32.
Craft sapling from 9 jungle saplings.

Use noise with 1 octave and flag 'absvalue' to create sand paths
in dunes.

Adjust selectionbox width for a better fit and consistency with other
grasses.

Remove flowers from coniferous forest.
Add 'dirt with coniferous litter' to farming mod overrides.

Original texture by tobyplowy, colourised by paramat.

Make deeper, to compensate for ease of travel using 3D noise tunnels.
Make depth more progressive with value:

Ore, upper limit

Coal 64 no change
Iron 0 no change
Tin -32 > -64
Copper -16 > -128
Gold -64 > -256
Diamond -128 > -512
Mese crystal -64 > -512
Mese block -1024 no change

Make y of 1st density increase deeper but following a similar logic to before:

Ore, upper limit

Coal 0 no change
Iron -64 no change
Tin -128 no change
Copper -64 > -256
Gold -256 > -1024
Diamond -256 > -1024
Mese crystal -256 > -1024
Mese block -2048 new

Add second, denser layer of mese blocks with a decrease of separation and
increase in density equal to mese ore layers.

Make the placeholder biomes grassland to make the shape of the floatland
terrain more visible.

Only register floatland biomes if mgv7 'biomerepeat' flag is false.
Simplify floatland biomes to coniferous forest and ocean.
Make 'mgv7_floatland_level' and 'mgv7_shadow_limit' parameters global
values for mods to use to register their own floatland biomes.

This allows us to preserve mgv6 ore distribution while giving us the freedom
to alter ore distribution for other mapgens.
Other mapgens are larger scale and have 3D noise tunnels which make vertical
travel easier, so ores can be deeper.
Other mapgens have registered biomes which allows us to limit ores to certain
biomes.

Update biome lists for blob ores.

Previously i had matched ore density to moreores, but this density was
too high and out of balance with MTGame, reduce density to be similar
to copper ore.
The 2 levels of underground ore were overlapping, fix.
Textures are actually by kilbith, update credits.

Also credit kilbith for bronze, copper, gold and steel blocks, credits
had not been updated at the time.

Use Calinou's textures from moreores mod.
Craft bronze from tin and copper instead of steel and copper.
Match ore density to the moreores mod but start ore at a depth of
y = -32 to be part of the depth progression of other ores.

A simple set of biomes for now: Ocean, coniferous forest, grassland,
sandstone desert.
Update biome lists for blob ores and decorations.

Make sandstone desert filler 1 node deeper to match other deserts.

Currently jungletrees and junglegrass use sidelen 80 for simplicity,
but this results in a more uneven distribution of decorations. A more
even distribution helps keep rainforest darker with a more unbroken
canopy.
This is also more consistent. 80 is based on the default mapchunk
size, all other decorations use sidelen 16 or smaller to divide into
any mapchunk size.