Place schematic (on vmanip): Enable use of 'place center' flags

[paramat]

Place schematic (on vmanip): Enable use of 'place center' flags

For 'place schematic' and 'place schematic on vmanip' APIs.
Fix 'place center' code to properly centre schematics.
Fix some comments.
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For #7066
@Gael-de-Sailly please could you test this?

[gaelysam]

I've tested your changes with a quickly written test mod that generates one pine tree per mapgen chunk.

local vmanip = true
local schem = minetest.get_modpath("default") .. "/schematics/pine_tree.mts"
local flags = "place_center_x,place_center_z"

minetest.register_on_generated(function(minp, maxp, seed)
	local pos = {x=math.floor((minp.x+maxp.x)/2), y=math.floor((minp.y+maxp.y)/2), z=math.floor((minp.z+maxp.z)/2)}
	if vmanip then
		local vm = minetest.get_mapgen_object("voxelmanip")
		minetest.place_schematic_on_vmanip(vm, pos, schem, "random", nil, false, flags)
		vm:write_to_map()
	else
		minetest.place_schematic(pos, schem, "random", nil, false, flags)
	end
	minetest.set_node(pos, {name="default:stone"})
end)

I expected the tree to be placed at (7,7,7), on the stone block, but it's placed at (6,7,6). As far as I know the pine tree schem doesn't have extra air rows that makes the trunk not being at the center, so it's misplaced.

By the way I did the same changes for minetest.place_schematic, it behaves exactly the same.

[gaelysam]

I just found the cause of the issue and fixed it in my branch.

[paramat]

Correct, see https://github.com/minetest/minetest/blob/master/src/mapgen/mg_decoration.cpp#L363

	if (flags & DECO_PLACE_CENTER_X)
		p.X -= (schematic->size.X - 1) / 2;
	if (flags & DECO_PLACE_CENTER_Z)
		p.Z -= (schematic->size.Z - 1) / 2;

I will do this for 'place schematic' too.

[paramat]

Updated.
@Gael-de-Sailly since i'm busy could you test this updated PR, testing 'place schematic' too? Also please test nothing breaks when there is no 'flags' field used (for backwards compatibility).
Once tested i can probably merge this as trivial.

[gaelysam]

All of this works. Both functions work as expected, and there is no bug when flags is omitted. I think you can merge.

[paramat]

Thanks.

[paramat]

c610643