This is similar to some other datablocks. Mainly applies to collections,
so freeing scene does not involve changing any non-directly owned data.
There are two main usecases foreseen for the future:
- Less CPU ticks on bmain free, where everything is freed anyway and
there is no need to preserve id counters.
- Easier freeing of temporary data, including data which is used by
depsgraph's copy-on-write mechanism.
Neither of those are currently implemented, but will be shortly.
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