micro-optimize bullet details

Shots that miss are most likely to not be there yet, so check Y-top first.

If I assume shots will never be larger than 16 pixels per side, there is
no reason to check the ship's genuine width and height.

vacuum_gambit.p8

@@ -775,7 +775,7 @@ end
 function bullet_base:move()
  local x,y = self.x + self.dx, self.y+self.dy
  self.x,self.y=x,y
- return (y<=128) and (y >= -(self.height<<3)) and (x <= 128) and (x >= -(self.width<<3))
+ return (y<=128) and (y >= -16) and (x <= 128) and (x >= -16)
 end
 
 function bullet_base:draw()
@@ -1352,10 +1352,10 @@ end
 -- box: x1, y1, x2, y2
 function collides(b1, b2)
 	return
-	  b1.x1<=b2.x2
+   b1.y1<=b2.y2
-	  and b1.y1<=b2.y2
-	  and b1.x2>=b2.x1
 	  and b1.y2>=b2.y1
+	  and b1.x1<=b2.x2
+	  and b1.x2>=b2.x1
 end
 
 collider = mknew{