Update screenshake position in fixed-timestep loop

Otherwise the screen will shake too fast for my liking.

Also I'm planning to add an FPS limiting option later (because right
now, un-capping the FPS is pretty wasteful and eats up lots of
resources, especially since I have only a 60hz monitor), and it'd feel
weird if screen shaking updated every delta timestep.

desktop_version/src/main.cpp

@@ -471,6 +471,7 @@ int main(int argc, char *argv[])
             if (game.infocus && game.screenshake > 0)
             {
                 game.screenshake--;
+                graphics.updatescreenshake();
             }
 
             //We did editorinput, now it's safe to turn this off