Merging core pull requests to upstream

Requirements

There are five major requirements that each pull request must fullfill in order to be mergeable to upstream Minetest. There exists no valid reason for anything to be merged without all of these passing.

1. It must follow a roadmap in some way, to make sure it fits the whole picture of the project. The most up-to-date, reliable, feasible and/or authoritative roadmap available today is probably celeron55's. [1]
2. It must work in the first place. Compile it and test it in game, or write mod code that uses it.
3. The code style must be correct. [2]
4. The internal interfaces of the code must be good, and it should be reasonably optimized, depending on how often the code is called.
5. The protocols and formats that it uses must be well designed, including the required compatibility in the part in question. On-disk formats are extremely important to get right. Modding API concerns are split between this and req2. This is about knowing the engine's design along with its pitfalls.

[1] https://forum.minetest.net/viewtopic.php?f=7&t=9177&p=139455 [2] http://dev.minetest.net/Code_style_guidelines

...which can be checked by:

1. Anyone.
2. Anyone, or at least any modder.
3. Those with a bit of general C++ and Lua knowledge.
4. Those with a lot of general C++ and Lua knowledge.
5. Those who have studied the file formats, the protocol and the Lua API, and know how version compatibility can be reliably done.

I (celeron55) am interested in seeing whether we could end up in a situation where pull requests would get comments like "req123 checked.", after which someone could do the rest and ultimately anyone could merge it based on these simple comments, after each of the five requirements have been checked.