Update 3rdparty/osgXR to version 0.5.0, which primarily gets us build
fixes for Windows. Unfortunately one of them requires an API breakage to
avoid some apparent preprocessor namespace pollution on Windows, which
will require minor source modification in FlightGear (left to the next
commit). The ABI is unchanged so binary compatibility is unaffected.
Update 3rdparty/osgXR to version 0.3.8, which gets us non-linear sRGB
swapchain formats by default. This avoids double gamma correction with
Monado when FlightGear renders non-linear sRGB data into linear RGB
swapchain images.
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.
