CMake configure based OpenImageIO

[mikaelsundell]

@jstone-lucasfilm and @lgritz: Accidentally deleted the previous PR after sync fork, sorry. Here is a fresh one with all changes. Kept the "plain" signature for target_link_libraries, need to change the previous target_link_libraries (33) to use keywords for targets MaterialXGenShader + ${CMAKE_DL_LIBS}, ok?

Deleted FindOpenImageIO.cmake + cleaned up a few of the non used variables, also fixed the deprecated call to read_image to include subimage and miplevel (2.0).