disallow idle animations while in the air paradropping

fixes OpenRA#2988

OpenRA.Mods.RA/Render/RenderInfantry.cs

@@ -49,7 +49,7 @@ protected virtual string NormalizeInfantrySequence(Actor self, string baseSequen
 
 		protected virtual bool AllowIdleAnimation(Actor self)
 		{
-			return info.IdleAnimations.Length > 0;
+			return info.IdleAnimations.Length > 0 && self.CenterPosition.Z == 0; // TODO: this will need to change for heightmaps
 		}
 
 		public AnimationState State { get; private set; }