ilmbase/openexr: updated to 2.3.0

OpenEXR v2.3.0 has been released and is available for download.

Features/Improvements:
ThreadPool overhead improvements, enable custom thread pool to be registered via ThreadPoolProvider class
Fixes to enable custom namespaces for Iex, Imf
Improve read performance for deep/zipped data, and SIMD-accelerated uncompress support
Added rawPixelDataToBuffer() function for access to compressed scanlines
Iex::BaseExc no longer derived from std::string.
Imath throw() specifiers removed
Initial Support for Python 3

Bugs:
25+ various bug fixes (see detailed Release Notes for the full list)

Build Fixes:
Various fixes to the cmake and autoconf build infrastructures
Various changes to support compiling for C++11 / C++14 / C++17 and GCC 6.3.1
Various fixes to address Windows build issues
60+ total build-related fixes (see detailed Release Notes for the full list)

(adam)