When boolean fields are evaluated and used as selections, we create
a vector of indices. This process is currently single-threaded, but
226f0c4fef7e7792c added a more optimized multi-threaded version
of this process. It's simple to use this in the field evaluator.

I tested this with the set position node and a random
value node set to boolean mode on a Ryzen 2700x:

In the future there could be a specialized version for non-span
virtual array selections that uses materialize to lower virtual
call overhead.

Tests pass locally.