Open Image Denoise v1.2.0

• Added neural network training code
• Added support for specifying user-trained models at runtime
• Slightly improved denoising quality (e.g. less ringing artifacts, less
  blurriness in some cases)
• Improved denoising speed by about 7-38% (mostly depending on the compiler)
• Added OIDN_STATIC_RUNTIME CMake option (for Windows only)
• Added support for OpenImageIO to the example apps (disabled by default)
• Added check for minimum supported TBB version
• Find debug versions of TBB
• Added testing