Some platforms (*cough* web *cough*) have hard limits on the number of
threads that can be spawned.
Currently, ThreadPoolWork (mostly used in rendering/physics servers)
will spawn as many threads as CPUs available causing exception on
machines with high CPU count.
This commit adds a new overridable method to OS that returns the default
thread pool size (still the CPU count by default), and overrides it for
the JavaScript platform so it always allocate only one thread.
We can likely improve the whole ThreadPoolWork in the future to always
allocate X amount of threads, and assign jobs to them on the fly, but
that will require some more architectural changes.
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