Better integrate with OS-level support for HDR displays

[doug-walker]

OCIO has support for using an ICC monitor profile to instantiate a "virtual display" that may be added to a config and used with a specific set of views. However, as operating systems such as macOS have added support for HDR (aka XDR) display technology, they have been using proprietary methods that do not rely on ICC profiles. (This is partly because the ICC format does not really support HDR monitors.)

As part of this transition to HDR displays, it has become less and less possible to bypass OS-level color management. So rather than OCIO calculating the exact RGB code values that are sent to the monitor, increasingly the application must accept that the OS is going to be modifying the values. This implies that the host application needs to tag the images in accordance with whatever processing has been done by OCIO. Therefore an OCIO display color space will need to contain some extra information to allow the application to use the appropriate OS-specific tag.

Furthermore, the application may need to do some negotiation between the color space tags that are in the config and the color space tags supported by the host OS and the capabilities of the connected displays. This will requires some further additions to the OCIO API.