This matches the OpenXR usage: the array is the plural of the element type,
and the count is the singular element type plus "count" (usually CountOutput
because of the two-call idiom)
Includes fixes to other code to match API changes.
Use none shader when XRT_DISTORTION_MODEL_NONE is set.
Fall back to none distortion if mesh generation fails or unsupported
distortion model is set.
comp_render init updates comp_settings::distortion_model.
u_compute_distortion_mesh: Don't touch xdev::distortion::preferred,
just add XRT_DISTORTION_MODEL_MESHUV mesh to xdev::distortion::models.
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
