This was a hard decision, because going newer CUDA toolkit makes
rendering up to 5% slower. But on another hand, it solves major
speed regressions (up to 30%) with branched path tracing on a
top level cards.
Neither of those regressions have a meaningful and sane workaround
from the code itself.
Toolkit 6.5 could still be used, but it's no longer recommended one.
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