Move stereo/omnidirectional camera features to core extension(s)

[jeffamstutz]

This proposal seeks to move the omnidirectional camera and stereo rendering features to be core extensions. This is because they are niche features that implementations are likely to ignore if they don't already exist as an engine feature. Furthermore, the existence of these features may imply supported use cases that engines may not be interested in, such as VR. Thus moving them to be core extensions, this keeps a standardized interface while making them ultimately optional for implementers.

The proposed extension names are ANARI_KHR_CAMERA_OMNIDIRECTIONAL and ANARI_KHR_CAMERA_STEREO_RENDERING respectively.

[jeffamstutz]

This issue collapses into issue #20.