Allow multiple cave liquids in a biome definition

[paramat]

Although labelled a feature, this is an important step in de-hardcoding some aspects of mapgen.

We already have a biome-defined cave liquids feature, however only 1 cave liquid per biome can be defined, which cannot reproduce the current mix of water and lava caves.

This PR allows games to replace the hardcoded engine distribution of cave liquids with biome-defined cave liquids, while preserving the ability to have multiple cave liquids in a biome (such as water and lava).
Relative chances can be altered by repeated entries in the list.

If a biome defined cave liquid (or multiple) is defined, each entire cave uses that node (or a randomly chosen node from the list). Previously, biome-defined cave liquid was calculated per cave segment.

The change from per-segment to per-cave avoids every cave segment randomly choosing a node from the list, resulting in too-rapidly changing cave liquids within one cave. For example if the 2 nodes were water and lava, switching back and forth every few nodes would result in almost completely obsidian, no more lava lakes.

So the code for biome-defined liquid has moved from 'carveRoute()' (which generates each cave segment) to 'makeCave()' (which applies to the entire cave).

If no biome defined cave liquids are defined, the behaviour falls back to the old hardcoded behaviour.

There is a small performance benefit as biome is no longer being calculated for every cave segment.

This arose from discussion of MTG cave liquids:
minetest/minetest_game#2339 (comment)
minetest/minetest_game#2358

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Commit 2:
Don't place nodes if cave liquid is defined as 'air'

Avoids pointlessly placing huge numbers of "air" nodes when a cave liquid is defined to be air, for when no cave liquid is wanted.

[paramat]

Works but having every cave segment a different liquid is bad.
EDIT: Fixed.

[paramat]

PR also does this:

If a biome defined cave liquid (or multiple) is defined, the entire cave uses that node (or a randomly chosen node from the list).
If multiple cave liquids are defined in a list, this avoids every cave segment randomly choosing a node from the list, resulting in too-rapidly changing cave liquids within one cave. For example if the 2 nodes were water and lava, switching back and forth every few nodes would result in almost completely obsidian, no more lava lakes.

If no biome defined cave liquids are defined, the behaviour falls back to the old hardcoded behaviour.

There is a small performance benefit as biome is no longer being calculated for every cave segment.

[paramat]

With:

	minetest.register_biome({
		name = "grassland_under",
		node_cave_liquid = {"default:meselamp", "default:mese_post_light"},
		y_max = -256,
		y_min = -31000,
		heat_point = 50,
		humidity_point = 35,
	})

In that biome:

Each entire cave uses one randomly chosen node from the list.

[paramat]

I'm seriously stuck.
In testing this seems to work but there is an error shown and i can't seem to silence it.
2019-04-20 02:02:22: ERROR[Main]: NodeResolver: no more node lists

[paramat]

Error seems fixed.
It was caused by the fallback 'default biome' that needed to 'push back' a list size of 1 for "node_cave_liquid". See line comment.

[paramat]

This fixed the error message.

[paramat]

Tested with a biome with all fields used, mod code (mod 'depends' on default and stairs):

minetest.clear_registered_biomes()
minetest.clear_registered_ores()
minetest.clear_registered_decorations()

minetest.register_biome({
	name = "test",
	node_dust = "default:snow",
	node_top = "default:dirt_with_snow",
	depth_top = 1,
	node_filler = "default:dirt",
	depth_filler = 3,
	node_stone = "default:sandstone",
	node_water_top = "default:ice",
	depth_water_top = 1,
	node_water = "default:permafrost",
	node_river_water = "default:lava_source",
	node_riverbed = "default:sand",
	depth_riverbed = 2,
	node_cave_liquid = {"default:meselamp", "default:mese_post_light"},
	node_dungeon = "default:stone_block",
	node_dungeon_alt = "default:sandstone_block",
	node_dungeon_stair = "stairs:stair_stone_block",
	--vertical_blend = ,
	y_max = 31000,
	y_min = -31000,
	heat_point = 50,
	humidity_point = 50,
})

When node_cave_liquid is left out, caves will revert to old hardcoded behaviour (water and lava).

[SmallJoker]

This is a nice feature, but I'd like to test its functionality as soon I've got time for it.
Just to mention that this PR isn't forgotten.

[paramat]

Simple commit 2 added, see first post.

SmallJoker, dev is unbearably slow at the moment so i suggest not testing to speed up approval, you can trust my testing. I'm getting impatient and feel i should be partially trusted with mapgen stuff =)

[SmallJoker]

Works as expected.