Fix the game texture clearing during menu shaking

Because of how `blackout` works, screen shaking must clear the gameplay
buffer. `blackout` simply pauses rendering, so if the gameplay buffer
gets cleared, then the screen will just be black, otherwise it'll look
like the game is "frozen". VVVVVV only uses `blackout` during screen
shaking, so it works as intended. However, when reimplementing this
behavior in the move to using the SDL_Renderer system, I failed to
notice that since my implementation always clears the gameplay buffer
when shaking, if you open the menu during a shake, instead of seeing
gameplay during the transition animation, you only see black. This has
been fixed with a simple `game.blackout` check before clearing the
gameplay buffer.

desktop_version/src/Graphics.cpp

@@ -3173,7 +3173,7 @@ void Graphics::screenshake(void)
     clear();
 
     // Clear the gameplay texture so blackout() is actually black after a screenshake
-    if (game.gamestate == GAMEMODE)
+    if (game.gamestate == GAMEMODE && game.blackout)
     {
         set_render_target(gameplayTexture);
         clear();