Extend bone override capabilities

[appgurueu]

Revival of #10704 - rebased, loads of bugs fixed, code cleaned up. Closes #12294. Fixes the bone rotation interpolation aspect of #10572. Closes #13688

Implements bone overrides supporting position, rotation and scale, all of which can be relative and/or interpolated.

Ready for review. Extensive testing needed.

How to test

1. Use mods like character_anim or playeranim to verify that the old API still works.

2. Use the new, documented API. Example based on @LoneWolfHT's headanim:

   1. Use the /headanim chatcommand to toggle the override.

mods/headlessanim/init.lua:

local lastdir = {}

minetest.register_globalstep(function(dtime)
	for _, player in pairs(minetest.get_connected_players()) do
		local pname = player:get_player_name()
		local ldeg = -math.deg(player:get_look_vertical())
		if lastdir[pname] == nil then
			lastdir[pname] = 0
		end
		if lastdir[pname] then
			if math.abs(lastdir[pname] - ldeg) > 4 then
				lastdir[pname] = ldeg
				player:set_bone_override("Head", {rotation = {vector = {x = ldeg, y = 0, z = 0}, absolute = true },
				scale = {vector = { x = 1.5, y = 1.5, z = 1.5 }}})
			end
		end
	end
end)

minetest.register_on_leaveplayer(function(player)
	lastdir[player:get_player_name()] = nil
end)

minetest.register_chatcommand("headanim", {
	func = function(name)
		local player = assert(minetest.get_player_by_name(name))
		if lastdir[name] then
			lastdir[name] = false -- don't update Head in globalstep
			player:set_bone_override"Head" -- clear override
		else
			lastdir[name] = 0
		end
	end
})

[SmallJoker]

The idea is good, but the code generally needs more comments and better pointer handling.

[appgurueu]

Thanks for the thorough review @SmallJoker! Everything should be addressed now, making the code noticeably cleaner and leading me to squish some bugs along the way!

[SmallJoker]

Sorry for the separate reviews. Taking a pause did however help to find a few more, although rather minor issues.

[appgurueu]

Rebased.

[appgurueu]

Bumped everything for 5.9.0 since we're in feature freeze for 5.8.0 now.

[appgurueu]

Thanks for the review. Should all be addressed now.

[MisterE123]

Wait, so legboat can actually walk smoothly now? As in, real, interpolated, terrain-based IK is possible?

[appgurueu]

After a bug report, I realized that this PR introduced a behavioral change in get_bone_position: Before, it would give you back whatever you passed to set_bone_position. Now, it gives you an equivalent rotation (assuming Irrlicht's quaternion to euler angle and vice versa conversions are correct), which can break code which relied on it giving you the very euler angles you put in (such as code which interpolates a rotation manually).

The possibly affected mods and games are the following: https://content.minetest.net/zipgrep/5296f687-4e1c-4ae1-8520-0119d7940824/. Notably ElCeejo's mods are affected, as well as possibly the Mineclone family.

A simple workaround would be to keep the original euler angles around (which mods could also do themselves). Should I implement that?

[sfan5]

No opinion but it'd be best to track this in a new issue.