OpenImageIO 2.1.16.0

Release 2.1.16 (1 Jun 2020) -- compared to 2.1.15

• OpenEXR: Fix bug in the channel sorting order when channels are "X" and
  "Y" (was reversing the order by confusing "Y" for "luminance"). #2595
• Python: Fixed a bug that lost certain string arguments, especially when
  passing a TypeDesc as its string equivalent. #2587
• fmath: Very minor fix to OIIO::clamp(), shouldn't affect normal use with
  floats at all, but fixed a subtle quasi-bug in OSL. #2594
• TypeDesc has additional helpers of constexpr values TypeFloat2,
  TypeVector2, TypeVector4, TypeVector2i, TypePointer. #2592
• Build: The exported CMake config files now set cmake variable
  OpenImageIO_PLUGIN_SEARCH_PATH #2584
• Docs: improvements and fixes to broken page rendering.