vdisp/sdl2: if [WAYLAND_]DISPLAY undef, use KMSDRM

If neither DISPLAY nor WAYLAND_DISPLAY environment variable is defined,
set driver=KMSDRM by default in Linux.

src/video_display/sdl2.c

@@ -5,7 +5,7 @@
  * @author Martin Pulec     <pulec@cesnet.cz>
  */
 /*
- * Copyright (c) 2018-2023 CESNET, z. s. p. o.
+ * Copyright (c) 2018-2024 CESNET
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -718,6 +718,15 @@ static void *display_sdl2_init(struct module *parent, const char *fmt, unsigned
                 return NULL;
         }
 
+#ifdef __linux__
+        if (driver == NULL && getenv("DISPLAY") == NULL &&
+            getenv("WAYLAND_DISPLAY") == NULL) {
+                MSG(NOTICE, "X11/Wayland doesn't seem to be running (according "
+                            "to env vars). Setting driver=KMSDRM.\n");
+                driver = "KMSDRM";
+        }
+#endif // defined __linux__
+
         if (SDL_VideoInit(driver) < 0) {
                 MSG(ERROR, "Unable to initialize SDL2 video: %s\n",
                     SDL_GetError());