make marching ants trailer effect optional

This makes trcolor = bgcolor work even when bgcolor is nil. trcolor=nil always works to produce this behavior.

panel_extreme.p8

@@ -378,7 +378,7 @@ ants = {
  -- bgcolor=0,  --nil: transparent
  ldcolor=7,
  fgcolor=6,
- trcolor=5,
+ -- trcolor=5,  --nil: no trailer
  
  speed=2/60,
  offset=0,
@@ -412,8 +412,8 @@ function ants:draw()
 			rectfill(x,0,x,h,c)
 		end
 	end
-	if p < 1 then
  local c = self.trcolor
+	if p < 1 and c then
 		for x = x0-1+s,w,s do
 			rectfill(x,0,x,h,c)
 		end