This adds a driver for the Vive family of devices based on Philipp
Zabel's ouvrt.
The driver currently only handles the IMU and main board report streams,
but is able to acquire the JSON configuration and utilize it for IMU
calibration as well as distortion configuration.
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.
Defines:
* XRT_HAVE_[name] is a external dep indicator.
* XRT_DRIVER_[name] if a internal driver is built.
CMake variables:
* BUILD_WITH_[name] is a external dep indicator.
* BUILD_DRIVER_[name] tells if we should build a specific driver.
