It was necessary to add a list of xdevs to oxr_sdl2_hack_start and to
populate such list from its callees.
That includes sdl2_program.gui_program->xdevs which was not being filled
for the monado-service target.
Calling SDL_Init() in different threads in the same process crashes.
In this SDL2 code X11_XOpenDisplay() will be a null pointer on the second thread;
if (SDL_X11_LoadSymbols()) {
display = X11_XOpenDisplay(NULL);
This becomes a problem when monado starts the debug UI in a thread,
and then an OpenXR client application tries to initialize SDL.
Two consecutive SDL_Init() in the same thread is harmless though.