Each HMD driver now has to implement compute_distortion() which will be called
by the compositor implementation to generate a mesh (usually).
u_distortion_mesh contains implementations for the defaults (panotools, OpenHMD, vive).
Also adds compute_distortion function for Vive distortion
There are differences between OpenHMD and Panotools values, main differences for now:
* psvr has 5 pano coefficients, ohmd has 3
* psvr uses viewport size and lens center in pixels for distortion calculation, ohmd in meter
* psvr uses different distortion scaling than ohmd
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.
This patch has been spun of the rather massive amount of work for adding input
devices into Monado. The interfaces feels somewhat stable now so could do
with another pair of eyes on them, before too much work is done on st/oxr.
