The last remaining ERR_EXPLAIN call is in FreeType code and makes sense as is
(conditionally defines the error message).
There are a few ERR_EXPLAINC calls for C-strings where String is not included
which can stay as is to avoid adding additional _MSGC macros just for that.
Part of #31244.
For clarity, assign-to-release idiom for PoolVector::Read/Write
replaced with a function call.
Existing uses replaced (or removed if already handled by scope)
This allows more consistency in the manner we include core headers,
where previously there would be a mix of absolute, relative and
include path-dependent includes.
Using `misc/scripts/fix_headers.py` on all Godot files.
Some missing header guards were added, and the header inclusion order
was fixed in the Bullet module.
