Blender Git Loki

XR: View adjustments for variable viewer scale

This adjusts some calculations and visibility flags for XR viewports in
order to account for a possible scale factor in the XR view matrix.

This scale factor can be introduced via the XR session settings base
scale, which allows a viewer to begin their session at a specific
reference scale, or the XR session state navigation scale, which allows
a viewer to adjust their scale relative to the reference scale during
the session.

Reviewed by Severin as part of D11501, but requested to be committed
separately.