Update 3rdparty/osgXR to version 0.3.9, which gets us better handling of
quirks and window closure, and the ability to create quad composition
layers which will be particularly useful for the loading screen when
flightgear isn't so good at timely frame updates.
Import osgXR from https://github.com/amalon/osgXR master branch into
3rdparty, specifically commit b7e222775553b529018ac4b847353327c24ae5d4,
which is 0.3.7 with tweaks for building as a subproject in a
subdirectory.
This will allow VR support to be more conveniently built if not already
installed, without having to fetch yet another dependency.
