Blender Git Loki

Pose backup: convert from C to C++

Convert `pose_backup.c` (in C) to `pose_backup.cc` (in C++). This will
make future improvements easier. For now, it's the same code with just
some additional explicit casts (C++ doesn't allow implicitly casting
`void *`), `NULL` changed into `nullptr`, and some other simple changes.

No functional changes.