graphics/openexr: bug fix update to 3.1.2

Documentation now also available through
https://openexr.readthedocs.io/

Release Notes:
https://github.com/AcademySoftwareFoundation/openexr/releases/tag/v3.1.2

Detailed ChangeLog:
https://github.com/AcademySoftwareFoundation/openexr/blob/v3.1.2/CHANGES.md#version-312-october-4-2021

Now requires py-breathe, Sphinx, and Doxygen, to build documentation.