Add missing includes on Windows.
Include GLX only on Linux.
Move openxr_platform.h include to OpenXrContext.h. To make this
possible, certain names from GLX/X11 need to be undefined in order to be
now includable in OpenXrInput.h.
Add Overte e.V. copyright.
Co-authored-by: Lubosz Sarnecki <lubosz@gmail.com>