Document that object properties colors field is unused

[cx384]

This PR documents that the object property colors field is unused and removes the one case where it was used.

Old

This PR removes the colors field from object properties.

It is not (sufficiently) documented, quite useless and takes up space.

This is the only place where it gets used, so only for upright_sprite with glow < 0, and the default color is white, but I didn't notice any difference, so I completely removed it instead of defaulting it to white., readded it, but I'm not sure if is needed.
https://github.com/minetest/minetest/blob/c8b5e3b741390fb98bfa9cdbae56930239ea271a/src/client/content_cao.cpp#L1406-L1408

The documentation for glow says Value < 0 disables light's effect on texture color. and this sill works.

To do

This PR is Ready for Review.

How to test

core.register_entity("testentities:upright_sprite2", {
	glow = -5,
	initial_properties = {
		visual = "upright_sprite",
		textures = {
			"testentities_upright_sprite1.png",
			"testentities_upright_sprite2.png",
		},
	},
})

[sfan5]

I think hardware-coloring for entities isn't so far-fetched so I'd keep this property but just document clearly that it does nothing right now.

[appgurueu]

As a minor side note, removing this property is technically a breaking change: Mods could be relying on :get_properties().colors existing, and they could expect to get back what they put in via :set_properties{colors = ...}.

This seems unlikely to me since this doesn't do anything right now, but you never know. Maybe someone is abusing this to store a couple colors (which is a really odd, bad way to store that, but 🤷).

[cx384]

I think hardware-coloring for entities isn't so far-fetched so I'd keep this property but just document clearly that it does nothing right now.

Is this really something we want to add?
I thought it is not necessary, since you can just apply a texture modifier by changing the textures field.
Also, for object textures, I think it is likely that we will get something similar to the Tile definition of nodes (which includes hardware coloring).

The documentation for object properties says:

These properties are not persistent, but are applied automatically to the corresponding Lua entity using the given registration fields. Player properties need to be saved manually.

So I think it is clear enough that you can't use those to store your own data. There could always be someone abusing unused things, e.g. if you don't use a _ prefix for a custom field when registering a node.

[appgurueu]

Is this really something we want to add?

Maybe. The problem with how texture modifiers work is that it requires creating and processing a new texture. Hardware coloring could be more efficient since all you have to do is pass a bunch of colors to a shader.

Arguably we could probably implement hardware coloring as a special optimization for texture modifiers here, so something like "if it's simply some base texture with a [multiply modifier, then turn that into hardware coloring applied to the base texture instead".

So I think it is clear enough that you can't use those to store your own data.

Well, it seems to me that the way the docs are written, and the way the code behaves, we are guaranteeing that this property exists, that you can put colors in there and get them back out, same as for the other properties, while the entity is active (whether the entity is persisted or not; "Store" doesn't mean "store persistently" here).

Modders shouldn't be doing that, since for entities using a custom field, say self._colors, is strictly better, and for players you should use a table indexed by player name, but I've seen modders do stuff they shouldn't be doing but which works (for example storing data that should not be persisted in player meta because it's convenient).

Though as said, this is in my opinion very minor. I wasn't sure whether I should've brought it up at all.

[cx384]

I think hardware-coloring for entities isn't so far-fetched so I'd keep this property but just document clearly that it does nothing right now.

Done.

@sfan5

[sfan5]

Fine by me but not sure if this qualifies as a bug fix anymore