Remove ContrailFader when complete. Fixes #4302.

OpenRA.Game/Graphics/ContrailRenderable.cs

@@ -15,6 +15,8 @@ namespace OpenRA.Graphics
 {
 	public struct ContrailRenderable : IRenderable
 	{
+		public int Length { get { return trail.Length; } }
+
 		readonly World world;
 
 		// Store trail positions in a circular buffer

OpenRA.Mods.RA/Effects/ContrailFader.cs

@@ -18,6 +18,7 @@ namespace OpenRA.Mods.RA.Effects
 	{
 		WPos pos;
 		ContrailRenderable trail;
+		int ticks;
 
 		public ContrailFader(WPos pos, ContrailRenderable trail)
 		{
@@ -27,6 +28,9 @@ namespace OpenRA.Mods.RA.Effects
 
 		public void Tick(World world)
 		{
+			if (ticks++ == trail.Length)
+				world.AddFrameEndTask(w => w.Remove(this));
+
 			trail.Update(pos);
 		}