Fix sound and particlespawner id generation

[Desour]

#14045 caused some issues:

• Server: avoid re-use of recent ParticleSpawner and Sound IDs #14045 (comment)
• Particlespawner ids were always 0.

With this PR, ids always grow, and don't return the last returned id. This is important, because the last returned id is likely to still be kept in lua.

cc @SmallJoker

To do

This PR is a Ready for Review.

How to test

• Sound ids: Server: avoid re-use of recent ParticleSpawner and Sound IDs #14045 (comment)
• Particlespawner ids: /lua local p=me:get_pos(); return core.add_particlespawner({time=10,minpos=p:offset(0,1,0),maxpos=p,texture="heart.png",amount=20})

[SmallJoker]

Now that you properly fixed it, I now can see what was meant with

This cloud is "smooth" and has no obvious gaps.

The cloud did move around without gaps, but was much shorter as compared to the proper fix. I should have double-checked (e.g. printed) the generated ID's to make sure that my PR actually works instead of relying on a visual test.

Tested this PR with the following change to the original script in #14044:

		print(minetest.add_particlespawner({
				time = 1,
				amount = 50,
				minexptime = 0.5,
				maxexptime = 0.5,
				minpos = vector.offset(targ, -0.5, -0.5, -0.5),
				maxpos = vector.offset(targ, 0.5, 0.5, 0.5),
				minsize = 1,
				maxsize = 1,
				texture = "[combine:1x1^[noalpha",
			}), "black")
	end
...
				maxsize = 1,
				texture = "[combine:1x1^[noalpha^[invert:rgb",
			})
		print(oldid, "white")
	end
	minetest.after(0, fx)

Producing the output

1	white
2	black
3	white
4	white
5	black
6	white
7	white
8	white
9	black
10	white

Which produces the numbers as desired. Thank you very much for catching this issue so early.

[Zughy]

@SmallJoker is your comment intended as an approval?

[SmallJoker]

@Zughy I intended to wait so that someone else could double-check it for sanity - after all I was too quick merging mine, so I did not want that to happen again.