This change implements dynamic loading of the OpenXR Loader library
on Android. If an OpenXR Loader library is not found,
Godot will still function with OpenXR disabled.
Also, on every platform, the OpenXR symbols are resolved at runtime
using xrGetInstanceProcAddr.
On Windows and Linux the OpenXR loader is included in the main
engine binary.
On Android, the OpenXR Loader is not built with the engine. Separately
distributed Android plugins will be provided with the correct loader
library for each device.
Co-authored-by: Gábor Pál Korom <gabor.p.korom@migeran.com>
Co-authored-by: Gábor Koncz <gabor.koncz@migeran.com>
Now the `linuxbsd` platform can be built headlessly (e.g. without X11
development libraries).
I also cleaned up some weird (old?) usages of the `env` variable which
seem to make no difference and are used nowhere else.
