This avoids recalculation of looptri derived triangulation whenever
switching to sculpt mode or whenever the PBVH is rebuilt, which can
happen after strokes in some situations. In my tests actually building
the PBVH is much more expensive (300ms), but this saves 6ms when
switching to sculpt mode and in other situations.
The cost is the possibility of higher memory usage because the cache
will live in the original main database mesh. However, the impact of
that will be smaller when the shared cache concept from D16204 is
applied to this data too.
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