math/Imath: new port (replaces graphics/ilmbase)

Patch series to update OpenEXR and ports that depend on ilmbase/OpenEXR
coming up.
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Imath is a basic, light-weight, and efficient C++ representation of 2D and 3D
vectors and matrices and other simple but useful mathematical objects,
functions, and data types common in computer graphics applications, including
the "half" 16-bit floating-point type.

Imath also includes optional python bindings for all types and functions,
including optimized implementations of vector and matrix arrays.

Imath was originally developed at Industrial Light & Magic in the early 2000's
and was originally distributed as open source as a part of the OpenEXR project.

Imath continues to be maintained as a sub-project of OpenEXR, which is now a
project of the Academy Software Foundation. See the OpenEXR project's
GOVERNANCE.md for more information about how the project operates.

WWW: https://github.com/AcademySoftwareFoundation/Imath/