This commits adds some new, specific flags to further control ID
remapping process (like and option to skip user refcounting completely).
It also adds a new function to do 'raw' remapping, without any extra
post-processing, depsgraph tagging, etc. This is not used currently, but
will soon be needed by readfile post-processing code changes.
There is also some small cleanups and reorganization in that area of code,
the main noticeable change being the switch from a short to an int for
the flags controlling remapping code (using short here does not give
any benefit, and makes it harder to switch to integers when it becomes
necessary).
No change in behaviors are expected from this commit.
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