Realtime use case affordances

We'd discussed this in a few other meetings, but I'd like to formally request some affordances in OpenPBR for realtime use cases, where realtime could include:

1. Consoles
2. Mobile devices
3. Web

The primary things that come to mind:

1. It's difficult for many rasterizers to support some of the features like transmission and subsurface. It would be great to allow for the following:
- Language that says how free implementations are with regards to supporting different feature sets. Right now its not completely clear whether an implementation can approximate or elide features that aren't possible on a specific hardware target
- Suggestions for how to approximate features if full path traced support is not possible

2. Realtime renderers often desire ambient occlusion maps which is not something that OpenPBR supports. It would be great to have some kind of officially supported way to do occlusion for renderers that prefer that route.

No file, test, or entry point is named; start by reviewing the OpenPBR specification and the requested realtime targets: consoles, mobile devices, and web. Define what implementation flexibility, feature approximation guidance, and officially supported ambient occlusion should mean, then consider the work done only when the project agrees on concrete specification changes.

Written by the indexing model from the issue text.