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.
