IlmImf Thread should report an 'optimal' number ofthreads to use. about openexr

[karlrasche]

I ended up adding some platform specific core/processor counting over in #112, at least for POSIX/windows.

It only lives in the standalone code generator, dwaLookups.cpp, but I could try moving that function into IlmThread if there's interest?

[cary-ilm]

Looking into the OpenEXR issue backlog. @kdt3rd, did your reworking of IlmThread address this? Is this worth keeping as an open issue?

[kdt3rd]

w/ c++11, the need for this is reduced, as people can just use std::thread::hardware_concurrency. However, a good point was raised that if core count changes or memory architectures change, we may want to not just return the number of cores. As such, added a (static) ThreadPool::estimateThreadCountForFileIO(). If someone else is just using threadpool for it's abilities, they can do what they want, and probably should anyway if their host is any sort of reasonably complex.