* Read directly into ExrChannel, eliminate intermediate MultiViewChannelName * Pass multiple channels to EXR writer together in IMB_exr_add_channels * Avoid various channel name parsing by passing components separately * Simplify logic for writing multichannel and multiview metadata * Remove unused global EXR handle storage * No longer use void pointer for EXR handle. * Use blender::Vector and std::string. * Slightly reshuffle code so multipart support will have smaller diff. * Add various comments. Pull Request: https://projects.blender.org/blender/blender/pulls/146650