Clean up background drawing code

As #974 states, the lab background only ever uses the first generated
value from `backboxint`, so let's change it to a normal variable
instead of an array. Also, stars don't need their width/height set to
2 constantly... they never change in the first place, Terry!

desktop_version/src/Graphics.h

@@ -399,7 +399,7 @@ public:
     SDL_Rect backboxes[numbackboxes];
     int backboxvx[numbackboxes];
     int backboxvy[numbackboxes];
-    float backboxint[numbackboxes];
+    float backboxmult;
 
     int warpskip;