(Drivers/Video) Implement frontend signal handling in null video driver

Allows RetroArch to gracefully exit when receiving stop signals, e.g.
`SIGINT` or `SIGTERM`, when using the null video driver.

retroarch.c

@@ -410,6 +410,8 @@ static void *video_null_init(const video_info_t *video,
    *input       = NULL;
    *input_data = NULL;
 
+   frontend_driver_install_signal_handler();
+
    return (void*)-1;
 }
 
@@ -422,7 +424,7 @@ static bool video_null_frame(void *data, const void *frame,
 
 static void video_null_free(void *data) { }
 static void video_null_set_nonblock_state(void *a, bool b, bool c, unsigned d) { }
-static bool video_null_alive(void *data) { return true; }
+static bool video_null_alive(void *data) { return frontend_driver_get_signal_handler_state() != 1; }
 static bool video_null_focus(void *data) { return true; }
 static bool video_null_has_windowed(void *data) { return true; }
 static bool video_null_suppress_screensaver(void *data, bool enable) { return false; }