CMake Version and OpenEXR_ROOT Incompatibility

[Arbeyem]

The current CMake minimum version is set to 3.3. This causes CMake to use the OLD behavior for CMP0074. This prevents usage of variables such as 'OpenEXR_ROOT' to find OpenEXR < 3.0.

The simplest fix would be to update the minimum version to 3.12 to allow *_ROOT variables. FindOpenEXR.cmake could also be refactored to utilize a differently named variable (such as OpenEXR_ROOT_DIR, similar to FindCAF.cmake).