Includes fixes from Christoph Haag:
```
st/gui: Fix segfault when starting monado-gui without arguments
```
```
st/gui: Run gui_prober_select only in the modules that need a prober
This also speeds up starting up the monado-gui main menu.
```
Co-authored-by: Moses Turner <moses@collabora.com>
Co-authored-by: Jakob Bornecrantz <jakob@collabora.com>
Co-authored-by: Christoph Haag <christoph.haag@collabora.com>
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.
