Blender Git Loki

VR: Change how landmarks affect viewer pose

* Changing to a landmark moves the view exactly to it, rather than
keeping the current position offset.
* Disabling positional tracking moves the viewer back to the landmark
position.
This is a more predictable and practical way to use landmarks. See
feedback in T71347.

On the code side, I did some cleanup so the logic flow is more clear.

Note: This is entirely untested. I currently don't have access to a
device. There might be issues, tomorrow I'll hopefully get feedback.