Abandons the assumption that in oxr_system.xdevs[], index 0 is HMD,
1 is left controller, 2 is right controller.
Now to represent the dynamically assigned roles, oxr_system.role contains
the index for a device in oxr_system.xdevs[] for head, left and right.
This role assignment happens on the client side and currently can not be updated
from the server side.
Also adds an enum that device drivers set indicating allowed assignments
(many controllers are physically designed to be held in a specific hand).
This also adds support for configurations with only a HMD and a right controller.
In order to access multiple devices the prober found interface needs to
pass the length of the device list. This patch updates the found
interface in all drivers.
Not pretty - no filtering on tracking, coordinate system
probably a little bit wrong still, no hemisphere tracking,
but enough to show movement and resizing in hello_xr.
