math/Imath: update to v3.2.0 bugfix release

ChangeLog: https://github.com/AcademySoftwareFoundation/Imath/releases/tag/v3.2.0

Note this changes the .so versioning again, so I'll again need to
do additional cleanups because people don't just depend on Imath.
This is unneeded extra work except in the situation of OpenEXR
which is co-developed with Imath by the upstream maintainer,
Academy Software Foundation (ASWF).

I have no intentions of MFH'ing this due to the .so versioning change.