Renaming the concept of `Reference Points` to `Anchors`. The Unity script updater should convert any references to `XRReferencePointSubsystem`, `XRReferencePoint`, and `XRReferencePointSubsystemDescriptor` the next time the project is loaded into the Editor.
### New
Added `XROcclusionSubsystem` for managing occlusion textures, such as the human segmentation stencil and human segmentation depth on some iOS devices.
