Those were only shallow wrappers around `BKE_id_copy`, barely used (even
fully unused in some cases), and we want to get rid of those ID-specific
helpers for the common ID management tasks. Also prevents weird custom
behaviors (like `BKE_object_copy`, who was the only basic ID copy
function to reset user count of the new copy to zero).
Part of 71219.
Surrounding includes with an 'extern "C"' block is not necessary anymore.
Also that made it harder to add any C++ code to some headers, or include headers
that have "optional" C++ code like `MEM_guardedalloc.h`.
I tested compilation on linux and windows (and got help from @LazyDodo).
If this still breaks compilation due to some linker error, the header containing
the symbol in question is probably missing an 'extern "C"' block.
Differential Revision: https://developer.blender.org/D7653
This moves the `alembic`, `avi`, `collada`, and `usd` modules into a common
`io` directory.
This also cleans up some `#include "../../{somedir}/{somefile}.h"` by
adding `../../io/{somedir}` to `CMakeLists.txt` and then just using
`#include "{somefile}.h"`.
No functional changes.