By removing the extra complete check/remapping of ID pointers in undo
case, ebb5643e59 merely revealed how broken the 'undo_preserve' code of
Scene was.
It cumulated a flock of issues, all more or less related to ID pointers:
* 'source of truth' should be the _old_ data (toolsettings), not the new
one, since this is the one kept at the end of the process;
* In some cases, some paint data may exist in one, but not the other
of the two 'old' and 'new' toolsettings data.
* Since this data is preserved to its latest version accross undos, its
ID pointers can become completely unrelated to these read from the
undo memfile, _even when the Scene itself is detected as unchanged_!
This implies that:
+ undo_preserve code has to be called even when there is no liblinking
(when the ID is detected as unchanged and re-used 'as-is').
+ Using existing ID addresses to find/validate an ID pointer in
undo_preserve process is like playing Russian roulette - invalid
memory access and crash is guaranteed at some point or another.
Use `session_uuid` value instead to ensure a valid ID pointer is set
(or null in case none can be found).
NOTE: while these issues also exist in previous releases (including both
latest LTSs), they were hidden by the code later in `setup_app_data`,
preventing any crash to happen. So backporting this fix would be far too
risky for a very minimal benefit imho.
94e6ab6d71