OpenSubdiv: Optimize speed of topology refiner construction

Now the conversion code uses mesh element mapping to speed up lookups.
Gives really nice speed improvement here, but the cost is higher memory
usage during refiner construction.

On the dragon scene here topology refiner construction time goes down
from 5 seconds to around 0.01.

It's possible to reduce the memory footprint by allocating mapping in
stages (don't allocate all of them at once, but do it on demand only
and free them after they're not needed anymore).

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