Make thread-pool class compile on Windows.
Make thread-pool class compile on Windows.

It seems that ssize_t (not a typo) is a POSIX-introduced type, not a C++ standard type. As such, compilers such as MSVC have no obligation to recognize it. As it seems that, while ssize_t was meant to be size_t + 1 signalling value ('-1'), most systems just take the 'make it signed, but use the same bit-width' _anyway_, which means that ssize_t has half the max in most practical cases, which in turn makes ptrdiff_t a perfectly cromulent replacement.

done as part of CURA-9356 which may also resolve CURA-8369

Ultimaker/CuraEngine,Ultimaker/CuraEngine