This method starting being used in 079ca220e1,
which now triggers this warning from GCC 10:
```
./core/error_macros.h:151:25: error: comparison of unsigned expression in '< 0' is always false [-Werror=type-limits]
```
Which means that reduz' beloved style which we all became used to
will now be changed automatically to remove the first empty line.
This makes us lean closer to 1TBS (the one true brace style) instead
of hybridating it with some Allman-inspired spacing.
There's still the case of braces around single-statement blocks that
needs to be addressed (but clang-format can't help with that, but
clang-tidy may if we agree about it).
Part of #33027.
Using `clang-tidy`'s `modernize-use-default-member-init` check and
manual review of the changes, and some extra manual changes that
`clang-tidy` failed to do.
Also went manually through all of `core` to find occurrences that
`clang-tidy` couldn't handle, especially all initializations done
in a constructor without using initializer lists.
Also implemented decal atlas, so projectors and other stuff can be added.
Sidenote: Had to make RID hashable, so some unrelated includes changed
in order to include it in hashfuncs.h
- Replace unintended `%` with `&`
- `get_owned_list()`: make thread-safe and fix logic
- Apply same logic fix to the destructor
Previously, the determination of owned RIDs was wrong. For instance, it could skip owned items or include duplicates in the list.
Avoids the engine crashing at exit.
