Thanks, it works just fine now. This time I went to the 25th level. More complaints: sometimes when I shoot too many bullets it is really hard to read my current position, and when the background goes red-colored I often barely see enemy bullets, although it is good I can destroy them.
Yeah, maybe I should render the player's ship with subtractive blending, or give it a different color. Also recolor enemies' bullets so that they're brighter.