`BKE_nlastrip_find_active()` and `update_active_strip()` now do a more
thorough search for the active strip, by also recursing into
meta-strips.
There were some assumptions in the NLA code that the active strips is
contained in the active track. This assumption turned out to be false:
when there is a meta-strip, the active strip could be inside that, and
then it's not contained in `active_track.strips` directly.
Apart from the above, there are other situations in which the track
pointed to by `AnimData::act_track` does *not* contain the active strip
(even indirectly).
Both these cases can happen when the transform system is moving a strip
that's in tweak mode. Entering tweak mode doesn't just search for the
active track/strip, but also falls back to the last-selected ones. This
means that the track/strip pointers & flags can be out of sync with
what's being tweaked/transformed. Because of this, the assumption that
there is any relation between "active strip" and "active track" has been
removed from the `update_active_strip()` function.
All this searching could be prevented by passing the `AnimData` to the
code that duplicates tracks & strips. That way, when the active
track/strip is dup'ed, the `AnimData` pointers can be updated as well.
That would require more refactoring and thus could introduce other bugs,
so that's left for later.
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