This de-duplicates the screen effects rendering code by putting it
inside a function, Graphics::renderwithscreeneffects(), and using that
instead of copy-pasted code.
The code to decrement the timers for flashing and shaking is now handled
outside the game-gamestate case-switch, instead of having to be
duplicated inside each render function.
As a bonus, I made it so the timer decrements even if screen effects are
disabled. This is to prevent any theoretical situation where the timer
can "pile up" due to disabled screen effects not letting it tick down.
It looks like this may have been used earlier in development, judging
from the name, obviously, but right now it seems like it's used as an
error message if a main game level is asked for an invalid room (well,
only two of them - the Lab and Warp Zone). It should probably be
formalized into an error system, if we want to keep teststring, and also
people would never see it anyway because I don't think there's a
reliable and consistent way to trigger loading a non-existent room.
I have seen someone manage to load a non-existent Warp Zone room only
one time, but even then this teststring didn't pop up. So this
teststring doesn't even trigger in the right circumstances.
Also, when it does pop up, as far as I can tell it will stay onscreen,
which is kinda annoying. So I'm just removing this ancient relic from
the code.
