Bump maxthreads to 512 (#3484)
Bump maxthreads to 512 (#3484)

The current limit has been already hit when AMD released
second-generation Epyc processors (codename Rome) back in 2019. Top
models have 64 cores per CPU, or 128 threads. This means 2 socket
configuration has 256 threads.

Fourth generation Epyc processors (codename Genoa) are rumored to have
96 cores per CPU, or 192 threads. This means 2 socket configuration has
384 threads. While the CPUs have not been announced yet, it makes sense
to prepare for them in advance.
OpenImageIO/oiio,lgritz/oiio,lgritz/oiio,lgritz/oiio,OpenImageIO/oiio,lgritz/oiio,OpenImageIO/oiio,OpenImageIO/oiio