The code ensuring a valid liboverride hierarchy root ID pointer for all
liboverrides in a given Main would fail in some cases, and crash on
asserts (in Debug builds) or due to corrupted data later in code
(release builds).
The main issue fixed here is re-entry in case of dependency loops (a
same ID being checked more than once within the same chain of recursive
calls to ensure its hierarchy root is valid). Solved the usual way now,
using the intermediate 'PROCESSING' flag instead of setting immediately
the 'PROCESSED' one, when recursively processing the chain of
dependencies.
A second issue fixed in that code was that in some cases, it could leave
the invalid existing hierarchy root pointer unchanged, because it could
not find a 'valid enough' alternative.
NOTE: This data corruption is presumably caused by 'make local'
operations on liboverride hierarchy root IDs. This will be addressed as
a second commit.
For reference, issue is reproducible when opening
`/promo/splash/040_0010.lighting.splash.blend` from Pets SVN repo r3106.
c5e10920f9