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.