`Scene` coordinate system is y-up, right-handed

[tomcrane]

copied from https://github.com/IIIF/3d/issues/20#issuecomment-1488927451

• In general, this space conforms to expectations from Three.js
• An infinite three-dimensional space
• Floating-point coordinates define each of the three axes
• Center of the space is at 0,0,0
• Annotations (of any kind) can exist in this 3D coordinate system, with an associated position to be defined through other user stories.
• Three.js coordinate system is +Y up and right-handed (+Z pointing toward viewer)