Fix opaque black background pixels in explosion sprites.

In the explosion sprites, make black pixels fully transparent, which otherwise block parts of the scene, e.g. stars and ship. Also make all other pixels semi-transparent, which appeared nicer.

demos/shooter/expl.c

@@ -64,7 +64,10 @@ void generate_explosions(void)
             lp[0] = pal->rgb[c2].r * 255;
             lp[1] = pal->rgb[c2].g * 255;
             lp[2] = pal->rgb[c2].b * 255;
-            lp[3] = pal->rgb[c2].a * 255;
+            if (c2 > 0)
+               lp[3] = pal->rgb[c2].a * 127;
+            else
+               lp[3] = 0;
          }
       }