Clean up all leftover resources in cleanup()

If there's a resource that doesn't otherwise need to be cleaned up and
is still alive upon program shutdown, then it should go in cleanup().

This cleans up Screen, GraphicsResources, Graphics buffers, Graphics
tiles, and musicclass audio upon program shutdown.

Even we technically don't NEED to clean these resources up ourselves
(the kernel is going to get rid of all of it anyway, else it'd be a
security problem), I'm doing this because otherwise Valgrind will
complain about these, and then it'd be difficult to see which memory
leaks are real and which are just "well this isn't really a leak but you
haven't freed this thing when the process exited, and that's technically
what a memory leak is".

These are all resources whose cleanup functions can be safely called
even if they haven't initialized anything yet.

desktop_version/src/main.cpp

@@ -365,7 +365,13 @@ int main(int argc, char *argv[])
 
 static void cleanup()
 {
+    /* Order matters! */
     game.savestatsandsettings();
+    gameScreen.destroy();
+    graphics.grphx.destroy();
+    graphics.destroy_buffers();
+    graphics.destroy();
+    music.destroy();
     NETWORK_shutdown();
     SDL_Quit();
     FILESYSTEM_deinit();