Rotated displays from OpenHMD in SteamVR
This MR allows SteamVR to use direct mode on a WVR2 with a 90° left rotated display.
It also clarifies various comments and reworks the OpenHMD viewport calculations. Before, it calculated two 2560x720 viewports instead of 1280x1440 viewports.
This MR should be tested with Oculus Rift DK2, Oculus Rift S, LG 360 VR.