Update some textures in lua_api.md

[someone-aka-sum1]

Add the default image prefix where still missing, make a few other examples functional as well

[sfan5]

These are just examples so they don't necessarily need to match any existing textures in e.g. Minetest Game.

[someone-aka-sum1]

I understand, but this allows anyone who downloads minetest to try them out. Besides, other examples do use textures from the default game.

[wsor4035]

minetest game isnt even the default anymore, nor is it packaged with minetest anymore. perhaps if we wanted to suggest good texturenames, should name the examples something like modname_texturename.extension

[SmallJoker]

mymod_grass.png would be better than grass.png because it implies that you should adhere to the recommended texture naming convention.

[someone-aka-sum1]

I agree that it makes sense to specify the texture naming convention in the docs, but it also makes sense for examples themselves to use concrete, widely-available textures.

Using default textures in modifier examples enables (most) developers to use them verbatim in order to get a sense for the effect of those modifiers. It's not like a developer would know how to replace a template, but would be confused as to how to replace a concrete texture, right?

I must say I find this pushback of yours on a trivial edit such as this peculiar. After all, I'm not the first to use default textures for examples. Unless I'm missing something, using default textures seems logical to me

[appgurueu]

I understand why someone would have a problem with the current examples, since they seem to encourage bad practice.

However, I'm not a fan of the proposed changes for the very same reason. Minetest Game has no special status anymore. Unnecessary dependencies on default are a bad thing. Modders should be aware of where their textures are coming from; usually, modders will want to supply their own textures. It needs to be clear that it is (usually) not the engine's job to provide a texture, but rather that of a mod or game, which need not be Minetest Game. To a new modder, the default_ prefix will be just confusing and look like leftover cruft from an engine - game separation which has long been done.

I think making it clear that these names are placeholders, and encouraging following naming conventions by adding a mymod_ prefix as others have suggested, is the best way forward.

A simple s/default_/mymod_/g (restricted to the changed lines) should do the job.

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ps.

I must say I find this pushback of yours on a trivial edit such as this peculiar.

This line of argumentation leads nowhere: If this edit is trivial, the push"back" is as trivially unwarranted as the push"front".