Make boss explosions end in transparency

[tnibert]

Right now, when you kill the boss, he explodes and each of the explosions have a black square appended to the end of the explosion that stays on screen to give the appearance of chunks being taken out of the boss.

This has two unintended consequences.

When you fly the space ship over these chunks, the chunks are rendered over top of the space ship so the space ship disappears. In addition, working on implementing new art that does not feature a black background and the black squares do not integrate well into this.

Making the black square be a transparent square will solve these issues.