Based on the HMD_viewport branch.
Integrates the OpenVR library to provide full (positional + rotational) tracking and support for direct mode. Adds a button to start an OpenVR session to the properties panel in a 3d viewport.
Only tested building on win64_vc14 with Visual Studio 2017, other platforms will need changes to the cmake platform files. Only tested with a Vive, though I imagine it should work with other HMDs supported by OpenVR.
Requires a folder openvr in the lib folder with files include/openvr.h, lib/openvr_api.dll, lib/openvr_api.lib from the OpenVR SDK. Also needs SteamVR installed.
Based on what I've read on forums I'm not sure if OpenVR is compatible with Blenders license, I implemented this mostly out of personal interest for viewing models with full tracking. See also: D2133