Convention for HUD element z_index

[Wuzzy]

In 5.2.0, z_index was added to HUD elements to determine the rendering order. As many different mods are manipulating the HUD, they will use z_index.

However, there is no convention whatsoever as to what numbers to use. I feel we should come up with some convention (even a simple one). Otherwise, the mods will just spread the z_indexes all over the place, making it useless. Therefore, I suggest to discuss a convention, and when the discussion is complete, put it into lua_api.txt or at least the Dev Wiki as official recommendation.

Please discuss here:

https://github.com/minetest/minetest/issues/9606