Add utility function to detect if SDL is inside a sandbox

Refactor the previous sandbox check in a standalone function that also
includes Snap support.

Signed-off-by: Ludovico de Nittis <ludovico.denittis@collabora.com>

CMakeLists.txt

@@ -1509,6 +1509,7 @@ elseif(UNIX AND NOT APPLE AND NOT RISCOS AND NOT HAIKU)
     # Always compiled for Linux, unconditionally:
     list(APPEND SOURCE_FILES "${SDL2_SOURCE_DIR}/src/core/linux/SDL_evdev_capabilities.c")
     list(APPEND SOURCE_FILES "${SDL2_SOURCE_DIR}/src/core/linux/SDL_threadprio.c")
+    list(APPEND SOURCE_FILES "${SDL2_SOURCE_DIR}/src/core/linux/SDL_sandbox.c")
 
     # src/core/unix/*.c is included in a generic if(UNIX) section, elsewhere.
   endif()