Indicate modes when loading in to gameplay

If you load in to gameplay with invincibility mode, glitchrunner mode,
Flip Mode, or slowdown enabled, then there will be text displayed on
screen for a few seconds that says so.

This is to serve as a useful reminder. A common pitfall with using
invincibility is forgetting to turn it off when you don't want it
anymore. What usually happens is that players forget that they have it
on until they encounter a hazard. Now, they can realize it as soon as
they load in.

See #1091.

desktop_version/src/Game.cpp

@@ -361,6 +361,9 @@ void Game::init(void)
     old_skip_message_timer = 0;
     skip_message_timer = 0;
 
+    old_mode_indicator_timer = 0;
+    mode_indicator_timer = 0;
+
     setdefaultcontrollerbuttons();
 }